Sum for Duration

Implemented the `Sum` trait for `Duration`. Seems reasonable.

Version 1.16.0 (2017-03-16)
===========================

Language
--------

* Lifetimes in statics and consts default to `'static`. [RFC 1623]
* [The compiler's `dead_code` lint now accounts for type aliases][38051].
* [Uninhabitable enums (those without any variants) no longer permit wildcard
  match patterns][38069]
* [Clean up semantics of `self` in an import list][38313]
* [`Self` may appear in `impl` headers][38920]
* [`Self` may appear in struct expressions][39282]

Compiler
--------

* [`rustc` now supports `--emit=metadata`, which causes rustc to emit
  a `.rmeta` file containing only crate metadata][38571]. This can be
  used by tools like the Rust Language Service to perform
  metadata-only builds.
* [Levenshtein based typo suggestions now work in most places, while
  previously they worked only for fields and sometimes for local
  variables][38927]. Together with the overhaul of "no
  resolution"/"unexpected resolution" errors (#[38154]) they result in
  large and systematic improvement in resolution diagnostics.
* [Fix `transmute::<T, U>` where `T` requires a bigger alignment than
  `U`][38670]
* [rustc: use -Xlinker when specifying an rpath with ',' in it][38798]
* [`rustc` no longer attempts to provide "consider using an explicit
  lifetime" suggestions][37057]. They were inaccurate.

Stabilized APIs
---------------

* [`VecDeque::truncate`]
* [`VecDeque::resize`]
* [`String::insert_str`]
* [`Duration::checked_add`]
* [`Duration::checked_sub`]
* [`Duration::checked_div`]
* [`Duration::checked_mul`]
* [`str::replacen`]
* [`str::repeat`]
* [`SocketAddr::is_ipv4`]
* [`SocketAddr::is_ipv6`]
* [`IpAddr::is_ipv4`]
* [`IpAddr::is_ipv6`]
* [`Vec::dedup_by`]
* [`Vec::dedup_by_key`]
* [`Result::unwrap_or_default`]
* [`<*const T>::wrapping_offset`]
* [`<*mut T>::wrapping_offset`]
* `CommandExt::creation_flags`
* [`File::set_permissions`]
* [`String::split_off`]

Libraries
---------

* [`[T]::binary_search` and `[T]::binary_search_by_key` now take
  their argument by `Borrow` parameter][37761]
* [All public types in std implement `Debug`][38006]
* [`IpAddr` implements `From<Ipv4Addr>` and `From<Ipv6Addr>`][38327]
* [`Ipv6Addr` implements `From<[u16; 8]>`][38131]
* [Ctrl-Z returns from `Stdin.read()` when reading from the console on
  Windows][38274]
* [std: Fix partial writes in `LineWriter`][38062]
* [std: Clamp max read/write sizes on Unix][38062]
* [Use more specific panic message for `&str` slicing errors][38066]
* [`TcpListener::set_only_v6` is deprecated][38304]. This
  functionality cannot be achieved in std currently.
* [`writeln!`, like `println!`, now accepts a form with no string
  or formatting arguments, to just print a newline][38469]
* [Implement `iter::Sum` and `iter::Product` for `Result`][38580]
* [Reduce the size of static data in `std_unicode::tables`][38781]
* [`char::EscapeDebug`, `EscapeDefault`, `EscapeUnicode`,
  `CaseMappingIter`, `ToLowercase`, `ToUppercase`, implement
  `Display`][38909]
* [`Duration` implements `Sum`][38712]
* [`String` implements `ToSocketAddrs`][39048]

Cargo
-----

* [The `cargo check` command does a type check of a project without
  building it][cargo/3296]
* [crates.io will display CI badges from Travis and AppVeyor, if
  specified in Cargo.toml][cargo/3546]
* [crates.io will display categories listed in Cargo.toml][cargo/3301]
* [Compilation profiles accept integer values for `debug`, in addition
  to `true` and `false`. These are passed to `rustc` as the value to
  `-C debuginfo`][cargo/3534]
* [Implement `cargo --version --verbose`][cargo/3604]
* [All builds now output 'dep-info' build dependencies compatible with
  make and ninja][cargo/3557]
* [Build all workspace members with `build --all`][cargo/3511]
* [Document all workspace members with `doc --all`][cargo/3515]
* [Path deps outside workspace are not members][cargo/3443]

Misc
----

* [`rustdoc` has a `--sysroot` argument that, like `rustc`, specifies
  the path to the Rust implementation][38589]
* [The `armv7-linux-androideabi` target no longer enables NEON
  extensions, per Google's ABI guide][38413]
* [The stock standard library can be compiled for Redox OS][38401]
* [Rust has initial SPARC support][38726]. Tier 3. No builds
  available.
* [Rust has experimental support for Nvidia PTX][38559]. Tier 3. No
  builds available.
* [Fix backtraces on i686-pc-windows-gnu by disabling FPO][39379]

Compatibility Notes
-------------------

* [Uninhabitable enums (those without any variants) no longer permit wildcard
  match patterns][38069]
* In this release, references to uninhabited types can not be
  pattern-matched. This was accidentally allowed in 1.15.
* [The compiler's `dead_code` lint now accounts for type aliases][38051].
* [Ctrl-Z returns from `Stdin.read()` when reading from the console on
  Windows][38274]
* [Clean up semantics of `self` in an import list][38313]

[37057]: rust-lang/rust#37057
[37761]: rust-lang/rust#37761
[38006]: rust-lang/rust#38006
[38051]: rust-lang/rust#38051
[38062]: rust-lang/rust#38062
[38062]: rust-lang/rust#38622
[38066]: rust-lang/rust#38066
[38069]: rust-lang/rust#38069
[38131]: rust-lang/rust#38131
[38154]: rust-lang/rust#38154
[38274]: rust-lang/rust#38274
[38304]: rust-lang/rust#38304
[38313]: rust-lang/rust#38313
[38314]: rust-lang/rust#38314
[38327]: rust-lang/rust#38327
[38401]: rust-lang/rust#38401
[38413]: rust-lang/rust#38413
[38469]: rust-lang/rust#38469
[38559]: rust-lang/rust#38559
[38571]: rust-lang/rust#38571
[38580]: rust-lang/rust#38580
[38589]: rust-lang/rust#38589
[38670]: rust-lang/rust#38670
[38712]: rust-lang/rust#38712
[38726]: rust-lang/rust#38726
[38781]: rust-lang/rust#38781
[38798]: rust-lang/rust#38798
[38909]: rust-lang/rust#38909
[38920]: rust-lang/rust#38920
[38927]: rust-lang/rust#38927
[39048]: rust-lang/rust#39048
[39282]: rust-lang/rust#39282
[39379]: rust-lang/rust#39379
[`<*const T>::wrapping_offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset
[`<*mut T>::wrapping_offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset
[`Duration::checked_add`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_add
[`Duration::checked_div`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_div
[`Duration::checked_mul`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_mul
[`Duration::checked_sub`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_sub
[`File::set_permissions`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.set_permissions
[`IpAddr::is_ipv4`]: https://doc.rust-lang.org/std/net/enum.IpAddr.html#method.is_ipv4
[`IpAddr::is_ipv6`]: https://doc.rust-lang.org/std/net/enum.IpAddr.html#method.is_ipv6
[`Result::unwrap_or_default`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.unwrap_or_default
[`SocketAddr::is_ipv4`]: https://doc.rust-lang.org/std/net/enum.SocketAddr.html#method.is_ipv4
[`SocketAddr::is_ipv6`]: https://doc.rust-lang.org/std/net/enum.SocketAddr.html#method.is_ipv6
[`String::insert_str`]: https://doc.rust-lang.org/std/string/struct.String.html#method.insert_str
[`String::split_off`]: https://doc.rust-lang.org/std/string/struct.String.html#method.split_off
[`Vec::dedup_by_key`]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.dedup_by_key
[`Vec::dedup_by`]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.dedup_by
[`VecDeque::resize`]:  https://doc.rust-lang.org/std/collections/vec_deque/struct.VecDeque.html#method.resize
[`VecDeque::truncate`]: https://doc.rust-lang.org/std/collections/vec_deque/struct.VecDeque.html#method.truncate
[`str::repeat`]: https://doc.rust-lang.org/std/primitive.str.html#method.repeat
[`str::replacen`]: https://doc.rust-lang.org/std/primitive.str.html#method.replacen
[cargo/3296]: rust-lang/cargo#3296
[cargo/3301]: rust-lang/cargo#3301
[cargo/3443]: rust-lang/cargo#3443
[cargo/3511]: rust-lang/cargo#3511
[cargo/3515]: rust-lang/cargo#3515
[cargo/3534]: rust-lang/cargo#3534
[cargo/3546]: rust-lang/cargo#3546
[cargo/3557]: rust-lang/cargo#3557
[cargo/3604]: rust-lang/cargo#3604
[RFC 1623]: https://github.com/rust-lang/rfcs/blob/master/text/1623-static.md