log/slog: initial commit

log/slog: initial commit

The slog structured logging package.

This code was copied from the slog directory of the x/exp repo
at commit 642cacee5cc05231f45555a333d07f1005ffc287, with the
following changes:

- Change import paths.
- Delete unused files list.go, list_test.go.
- Rename example_depth_test.go to example_wrap_test.go and
  adjust example output.
- Change the tag safe_values to safe_slog_values.
- Make captureHandler goroutine-safe to fix a race condition
  in benchmarks.
- Other small changes as suggested in review comments.

Also, add dependencies to go/build/deps_test.go.

Also, add new API for the API checker.

Updates golang/go#56345.

Change-Id: Id8d720967571ced5c5f32c84a8dd9584943cd7df
Reviewed-on: https://go-review.googlesource.com/c/go/+/477295
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>