bytes.IndexByte is heavily optimized. Use it in findnull.
This is second attempt, similar to CL97523.
In this version we never call IndexByte on region of memory,
that crosses page boundary. A bit slower than CL97523,
but still fast:

name        old time/op  new time/op  delta
GoString-6   164ns ± 2%   118ns ± 0%  -28.00%  (p=0.000 n=10+6)

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

Fixes #23830

Change-Id: Id843dd4f65a34309d92bdd8df229e484d26b0cb2
Reviewed-on: https://go-review.googlesource.com/98015
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>