This patch adds the `Matcher` monad, which is capable of filtering any
recursive data structure, bottom-up, yielding a list of (or an
optional) result. These functions are probably going to be used over
`Term` values, so API is provided to wrap common projection functions.
The API was more or less copied directly from that of Clang's AST
matching facilities.
There are a lot of things we can do in the future:
* Binding results yielded in matchers to associated names, for future
transformation stages to look up and modify.
* Actual transformation stages.
* Optimizations. This is not very fast.
A million thanks to @robrix, whose sage advice managed to turn my
kooky idea for an API into something really special and exciting.
