Patterns on types: operations to dissect a composite type

[jonsterling]

We should have something like

type 'a pattern
val match_tp : D.tp -> 'a pattern -> 'a m

An example would be something like:

let* base, fam = match_tp tp PatPi in 
...

A priori these would just behave like ordinary pattern matching, but we would eventually be able to support doing things like going under splits for "stable" patterns.