work in progress, and we are not ready to freeze its API for Go 1.

Package html still exists, containing just two functions: EscapeString
and UnescapeString.

Both the packages at exp/html and html are "package html". The former
is a superset of the latter.

At some point in the future, the exp/html code will move back into
html, once we have finalized the parser API.

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/5571059