errors: deprecate functions that are now in standard library

Change-Id: I8dc2a0d3923fdd55bcc205eea99142b1a32d569d
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/399607
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
2 files changed