The marshal method allows the hash's internal state to be serialized and
unmarshaled at a later time, without having the re-write the entire stream
of data that was already written to the hash.

Fixes #20573

Change-Id: I40bbb84702ac4b7c5662f99bf943cdf4081203e5
Reviewed-on: https://go-review.googlesource.com/66710Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>