Add initial support for parsing patterns into parse trees

Introduce a parser for patterns that generates a parse tree. The
language understood by the parser is:

pattern = '?'TERM
        | '?'TERM '(' pattern (',' pattern)* ','? ')'
        | WORD
        | QUOTED-WORD
TERM    = [0-9a-zA-Z-]
WORD    = [0-9a-ZA-Z-.*^$\[\]_\\]
QUOTED_WORD = "..."		# you know what I mean

This language is context free, which is a massive simplification
from aptitude's language, where ?foo(bar) could have two different
meanings depending on whether ?foo takes an argument or not.

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