Unification of implementation of existing md5.Write function
with other implementations (sha1, sha256, sha512).

Change-Id: I58ae02d165b17fc221953a5b4b986048b46c0508
Reviewed-on: https://go-review.googlesource.com/16621Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>