This function is present in the strings package but missing from bytes,
and we would like to keep the two packages consistent.

Add it to bytes, and copy the test over as well.

Fixes #15140

Change-Id: I5dbd28da83a9fe741885794ed15f2af2f826cb3c
Reviewed-on: https://go-review.googlesource.com/21562Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>