bytes.IndexByte is heavily optimized.
Use it in findnull.

name        old time/op  new time/op  delta
GoString-8  65.5ns ± 1%  40.2ns ± 1%  -38.62%  (p=0.000 n=19+19)

findnull is also used in gostringnocopy,
which is used in many hot spots in the runtime.

Fixes #23830

Change-Id: I2e6cb279c7d8078f8844065de684cc3567fe89d7
Reviewed-on: https://go-review.googlesource.com/97523
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>