crypto: switch block ciphers to detination first.

crypto: switch block ciphers to detination first.

Previously all the functions took two arguments: src, dst. This is the
reverse of the usual Go style and worth changing sooner rather than
later.

Unfortunately, this is a change that the type system doesn't help
with. However, it's not a subtle change: any unittest worth the name
should catch this.

R=rsc, r
CC=golang-dev
https://golang.org/cl/2751042