Demonstrate an alternate approach to the "level" package offered via
the "github.com/go-kit/kit/log/experimental_level" import path,
allowing log event level filtering via instances of the level.leveler
interface, created by functions that specify the filtering threshold.

Features not accommodated here for now:
- Returning an error from log.Log when squelching an event
- Returning an error from log.Log for events that lack a level
- Documentation

Benchmarks demonstrate both run time and allocation reduction for the
disallowed/squelching cases compared to the "experimental_level"
alternate:

name                     old time/op    new time/op    delta
NopBaseline-8               459ns ± 0%     130ns ± 0%  -71.68%
NopDisallowedLevel-8        509ns ± 0%     136ns ± 0%  -73.28%
NopAllowedLevel-8           515ns ± 0%     494ns ± 0%   -4.08%
JSONBaseline-8             2.41µs ± 0%    0.13µs ± 0%  -94.45%
JSONDisallowedLevel-8       506ns ± 0%     136ns ± 0%  -73.12%
JSONAllowedLevel-8         2.49µs ± 0%    2.45µs ± 0%   -1.48%
LogfmtBaseline-8           1.08µs ± 0%    0.13µs ± 0%  -87.92%
LogfmtDisallowedLevel-8     507ns ± 0%     136ns ± 0%  -73.18%
LogfmtAllowedLevel-8       1.13µs ± 0%    1.10µs ± 0%   -3.26%

name                     old alloc/op   new alloc/op   delta
NopBaseline-8                288B ± 0%       64B ± 0%  -77.78%
NopDisallowedLevel-8         288B ± 0%       64B ± 0%  -77.78%
NopAllowedLevel-8            288B ± 0%      288B ± 0%   +0.00%
JSONBaseline-8               968B ± 0%       64B ± 0%  -93.39%
JSONDisallowedLevel-8        288B ± 0%       64B ± 0%  -77.78%
JSONAllowedLevel-8           968B ± 0%      968B ± 0%   +0.00%
LogfmtBaseline-8             288B ± 0%       64B ± 0%  -77.78%
LogfmtDisallowedLevel-8      288B ± 0%       64B ± 0%  -77.78%
LogfmtAllowedLevel-8         288B ± 0%      288B ± 0%   +0.00%

name                     old allocs/op  new allocs/op  delta
NopBaseline-8                9.00 ± 0%      3.00 ± 0%  -66.67%
NopDisallowedLevel-8         9.00 ± 0%      3.00 ± 0%  -66.67%
NopAllowedLevel-8            9.00 ± 0%      9.00 ± 0%   +0.00%
JSONBaseline-8               22.0 ± 0%       3.0 ± 0%  -86.36%
JSONDisallowedLevel-8        9.00 ± 0%      3.00 ± 0%  -66.67%
JSONAllowedLevel-8           22.0 ± 0%      22.0 ± 0%   +0.00%
LogfmtBaseline-8             9.00 ± 0%      3.00 ± 0%  -66.67%
LogfmtDisallowedLevel-8      9.00 ± 0%      3.00 ± 0%  -66.67%
LogfmtAllowedLevel-8         9.00 ± 0%      9.00 ± 0%   +0.00%

Introduce the "log.Delegate" function to extract a wrapped delegate
Logger instance from a decorating instance, allowing the level-related
functions to find the configuration they need within the outermost
Logger created by one of the leveled factory functions.

Benchmarks demonstrate both run time and allocation reduction for the
disallowed/squelching cases compared to the "experimental_level"
alternate:

name                     old time/op    new time/op    delta
NopBaseline-8               470ns ± 0%     494ns ± 0%   +5.11%
NopDisallowedLevel-8        513ns ± 0%     123ns ± 0%  -76.02%
NopAllowedLevel-8           516ns ± 0%     474ns ± 0%   -8.14%
JSONBaseline-8             2.44µs ± 0%    2.39µs ± 0%   -2.45%
JSONDisallowedLevel-8       515ns ± 0%     123ns ± 0%  -76.12%
JSONAllowedLevel-8         2.48µs ± 0%    2.33µs ± 0%   -5.97%
LogfmtBaseline-8           1.05µs ± 0%    1.03µs ± 0%   -2.19%
LogfmtDisallowedLevel-8     514ns ± 0%     121ns ± 0%  -76.46%
LogfmtAllowedLevel-8       1.13µs ± 0%    1.02µs ± 0%   -9.39%

name                     old alloc/op   new alloc/op   delta
NopBaseline-8                288B ± 0%      288B ± 0%   +0.00%
NopDisallowedLevel-8         288B ± 0%       64B ± 0%  -77.78%
NopAllowedLevel-8            288B ± 0%      288B ± 0%   +0.00%
JSONBaseline-8               968B ± 0%      968B ± 0%   +0.00%
JSONDisallowedLevel-8        288B ± 0%       64B ± 0%  -77.78%
JSONAllowedLevel-8           968B ± 0%      968B ± 0%   +0.00%
LogfmtBaseline-8             288B ± 0%      288B ± 0%   +0.00%
LogfmtDisallowedLevel-8      288B ± 0%       64B ± 0%  -77.78%
LogfmtAllowedLevel-8         288B ± 0%      288B ± 0%   +0.00%

name                     old allocs/op  new allocs/op  delta
NopBaseline-8                9.00 ± 0%      9.00 ± 0%   +0.00%
NopDisallowedLevel-8         9.00 ± 0%      3.00 ± 0%  -66.67%
NopAllowedLevel-8            9.00 ± 0%      9.00 ± 0%   +0.00%
JSONBaseline-8               22.0 ± 0%      22.0 ± 0%   +0.00%
JSONDisallowedLevel-8        9.00 ± 0%      3.00 ± 0%  -66.67%
JSONAllowedLevel-8           22.0 ± 0%      22.0 ± 0%   +0.00%
LogfmtBaseline-8             9.00 ± 0%      9.00 ± 0%   +0.00%
LogfmtDisallowedLevel-8      9.00 ± 0%      3.00 ± 0%  -66.67%
LogfmtAllowedLevel-8         9.00 ± 0%      9.00 ± 0%   +0.00%

The running time increase in the "NopBaseline-8" benchmare is due to
the level.Debug function doing more work, needing to inspect the
supplied log.Logger to see whether it wraps a level restriction that
would preclude preceding with creating a log.Context to record the
level.