Add a []byte argument to hash.Hash to allow an allocation to be saved.

This is the result of running `gofix -r hashsum` over the tree, changing
the hash function implementations by hand and then fixing a couple of
instances where gofix didn't catch something.

The changed implementations are as simple as possible while still
working: I'm not trying to optimise in this CL.

R=rsc, cw, rogpeppe
CC=golang-dev
https://golang.org/cl/5448065
49 files changed