match: add basic wrapper for boolean function
This serves as a generic wrapper for fileset predicates. In future patches,
a fileset expression will be mapped to a tree of matchers for a better support
of match attributes such as visitdir(). For example,
$ hg debugwalk -v 'set:contrib/** and binary()'
* matcher:
<intersectionmatcher
m1=<patternmatcher patterns='(?:contrib/.*$)'>,
m2=<predicatematcher pred=binary>>
...