First was, apart from adding tests, a single line of code (to add the
constructor function). Adding SHA512-based hashing to crypto/hmac
required minor rework of the package because of a previously hardcoded
block-size in it's implementation. Instead of using a hash.Hash
generator function the constructor function now uses a crypto.Hash
type, which was extended to expose information about block size.

The only standard library package impacted by the change is
crypto/tls, for which the fix is included in this patch. It might be
useful to extend gofix to include this API change too.

R=agl, r, rsc, r
CC=golang-dev
https://golang.org/cl/5550043