runtime: use staticuint64s instead of staticbytes for 1-length strings

This was the last remaining use of staticbytes, so we can now
delete it.

The new code appears slightly faster on amd64:

name                   old time/op  new time/op  delta
SliceByteToString/1-4  6.29ns ± 2%  5.89ns ± 1%  -6.46%  (p=0.000 n=14+14)

This may not be the case on the big-endian architectures, since they have
to do an extra addition.

Updates #37612

Change-Id: Icb84c5911ba025f798de152849992a55be99e4f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/221979


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>