Still TODO: parsing optimizations

make_perl_groups.pl is copied with minimal modifications
(just to generate Go syntax instead of C++) from RE2.
Google Inc is "The RE2 Author" of that file and is one of
the Go Authors, so copyright changed to the Go Authors instead.

R=sam.thorogood, r, fvbommel, robert.hencke
CC=golang-dev
https://golang.org/cl/4612041