Use the abstract interpreter

[Bike]

A while ago I wrote an abstract interpreter in Cleavir based on stuff in Cousot. The practical upshot is that it should provide a clean way to do actual fixpoint iteration, instead of the "try a few iterations and then give up" Cleavir does now.

While the interpreter is written, it' untested and may not be practically useful. What I am thinking is that we can interleave fixpoint iterations, which improve inferred information about the IR (e.g. types) without modifying it, with modifications to the IR based on that information.