match: replace match class by match function (API)
The matcher class is getting hard to understand. It will be easier to
follow if we can break it up into simpler matchers that we then
compose. I'm hoping to have one matcher that accepts regular
(non-include) patterns, one for exact file matches, one that always
matches (and maybe one that never does) and then compose them by
intersection and difference.
This patch takes a simple but important step towards that goal by
making match.match() a function (and renaming the matcher class itself
from "match" to "matcher"). The new function will eventually be
responsible for creating the simple matchers and composing them.
icasefsmatcher similarly gets a factory function (called
"icasefsmatch"). I also moved the other factory functions nearby.