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>