There are some basic tests in the packages implementing the hashes,
but this one is meant to be comprehensive for the standard library
as a whole.

Most importantly, it locks in the current representations and makes
sure that they do not change from release to release (and also, as a
result, that future releases can parse the representations generated
by older releases).

The crypto/* MarshalBinary implementations are being changed
in this CL to write only d.x[:d.nx] to the encoding, with zeros for
the remainder of the slice d.x[d.nx:]. The old encoding wrote the
whole d.x, but that exposed an internal detail: whether d.x is
cleared after a full buffer is accumulated, and also whether d.x was
used at all for previous blocks (consider 1-byte writes vs 1024-byte writes).
The new encoding writes only what the decoder needs to know,
nothing more.

In fact the old encodings were arguably also a security hole,
because they exposed data written even before the most recent
call to the Reset method, data that clearly has no impact on the
current hash and clearly should not be exposed. The leakage
is clearly visible in the old crypto/sha1 golden test tables also
being modified in this CL.

Change-Id: I4e9193a3ec5f91d27ce7d0aa24c19b3923741416
Reviewed-on: https://go-review.googlesource.com/82136
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>