compress/flate: use go generate to create fixedhuff.go

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/137750043

Committer: Rob Pike <r@golang.org>
crypto/md5: use go generate to create md5block.go

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/129650043

Committer: Rob Pike <r@golang.org>