regexp: use sync.Pool to cache regexp.machine objects

regexp: use sync.Pool to cache regexp.machine objects

Performance optimization for the internals of the Regexp type. This adds
no features and has no user-visible impact beyond performance. Copy now
shares the cache, so memory usage for programs that use Copy a lot
should go down; Copy has effectively become a no-op.

The before v. after benchmark results show a lot of noise from run to
run, but there's a clear improvement to the Shared case and no detriment
to the Copied case.

BenchmarkMatchParallelShared-4                        361           77.9          -78.42%
BenchmarkMatchParallelCopied-4                        70.3          72.2          +2.70%

Macro benchmarks show that the lock contention in Regexp is gone, and my
server is now able to scale linearly 2.5x times more than before (and I
only stopped there because I ran out of CPU in my test machine).

Fixes #24411

Change-Id: Ib33abff2802f27599f5d09084775e95b54e3e1d7
Reviewed-on: https://go-review.googlesource.com/101715
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>