internal/itoa, os: move os.uitox to itoa.Uitox

internal/itoa, os: move os.uitox to itoa.Uitox

This packages already contains other similar functions. Also add a test
for it.

Change-Id: Iafa8c14f5cb1f5ef89a0e16ccc855c568a3b5727
Reviewed-on: https://go-review.googlesource.com/c/go/+/518317
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>