builder for simple recursive-descent parsers

 Parser::MGC provides a low-level framework
 for building recursive-descent parsers
 that consume a given input string from left to right,
 returning a parse structure.
 It takes its name from the `m//gc' regexps
 used to implement the token parsing behaviour.
 .
 It provides a number of token-parsing methods,
 which each extract a grammatical token from the string.
 It also provides wrapping methods
 that can be used to build up a possibly-recursive grammar structure,
 by applying a structure around other parts of parsing code.