internal/zstd: new internal package for zstd decompression

internal/zstd: new internal package for zstd decompression

This package only does zstd decompression, which is starting to
be used for ELF debug sections. If we need zstd compression we
should use github.com/klauspost/compress/zstd. But for now that
is a very large package to vendor into the standard library.

For #55107

Change-Id: I60ede735357d491be653477ed419cf5f2f0d3f71
Reviewed-on: https://go-review.googlesource.com/c/go/+/473356
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>