impl: substantially reduce regex stack size

[BurntSushi]

This commit fixes a fairly large regression in the stack size of a Regex
introduced in regex 1.4.4. When I dropped thread_local and replaced it
with Pool, it turned out that Pool inlined a T into its struct and a
Regex in turn had Pool inlined into itself. It further turns out that
the T=ProgramCache is itself quite large.

We fix this by introducing an indirection in the inner regex type. That
is, we use a Box instead of a Pool. This shrinks the size of a
Regex from 856 bytes to 16 bytes.

Interestingly, prior to regex 1.4.4, a Regex was still quite substantial
in size, coming in at around 552 bytes. So it looks like the 1.4.4
release didn't dramatically increase it, but it increased it enough that
folks started experiencing real problems: stack overflows.

Since indirection can lead to worse locality and performance loss, I did
run the benchmark suite. I couldn't see any measurable difference. This
is generally what I would expect. This is an indirection at a fairly
high level. There's lots of other indirection already, and this
indirection isn't accessed in a hot path. (The regex cache itself is of
course used in hot paths, but by the time we get there, we have already
followed this particular pointer.)

We also include a regression test that asserts a Regex (and company) are
16 bytes in size. While this isn't an API guarantee, it at least means
that increasing the size of Regex will be an intentional thing in the
future and not an accidental leakage of implementation details.

Fixes #750, Fixes #751

Ref servo/servo#28269

[BurntSushi]

This PR is on crates.io in regex 1.4.5.