Move bytes.Count and strings.Count to bytealg.

Update #19792

Change-Id: I3e4e14b504a0b71758885bb131e5656e342cf8cb
Reviewed-on: https://go-review.googlesource.com/98495
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>