Rakudo parse grammar

Files: src/parser/grammar.pg, src/parser/quote_expression.pir

Based on "STD.pm" grammar started by Larry Wall in Feb 2007 (http://svn.pugscode.org/pugs/src/perl6/STD.pm)

STD.pm uses "proto regexes" and other new constructs

Rakudo approximates STD.pm for now

Plan: Rakudo's grammar and STD.pm to converge