Parser is a work in progress but can populate most of the
interesting parts of the data structure, so a good checkpoint.
All the complicated Perl syntax is missing, as are various
important optimizations made during parsing to the
syntax tree.

The plan is that exp/regexp's API will mimic regexp,
and exp/regexp/syntax provides the parser directly
for programs that need it (and for implementing exp/regexp).

Once finished, exp/regexp will replace regexp.

R=r, sam.thorogood, kevlar, edsrzf
CC=golang-dev
https://golang.org/cl/4538123