exp/template: split the parse tree into a separate package exp/template/parse

exp/template: split the parse tree into a separate package exp/template/parse

Mostly a mechanical change, with a few cleanups to make the split easier.
The external interface to exp/template is unaffected.

In another round I will play with the function map setup to see if I can
avoid exposing reflect across the boundary, but that will require some
structural changes I did not want to mix into this CL.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4849049