bytes, strings: restore O(1) behavior of IndexAny(s, "") and LastIndexAny(s, "")

CL 65851 (bytes) and CL 65910 (strings) “improve[d] readability”
by removing the special case that bypassed the whole function body
when chars == "". In doing so, yes, the function was unindented a
level, which is nice, but the runtime of that case went from O(1) to O(n)
where n = len(s).

I don't know if anyone's code depends on the O(1) behavior in this case,
but quite possibly someone's does.

This CL adds the special case back, with a comment to prevent future
deletions, and without reindenting each function body in full.

Change-Id: I5aba33922b304dd1b8657e6d51d6c937a7f95c81
Reviewed-on: https://go-review.googlesource.com/78112


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>