html/template: update to new template API

Not quite done yet but enough is here to review.

Embedding is eliminated so clients can't accidentally reach
methods of text/template.Template that would break the
invariants.

TODO later: Add and Clone are unimplemented.
TODO later: address issue 2349

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5434077
5 files changed