Add a mechanism for debugging the abstract interpretation code

[travitch]

Right now, debugging this code is tricky: one generally has to insert Debug.Trace.trace calls to observe where the abstract interpretation breaks down when bugs are observed. There is some infrastructure with a debug function in the module, but its application is haphazard (i.e., the current probes are often not where you want them). It also needs to be documented to make it clear when/how to activate it.

We also may want to reconsider the current debugging infrastructure, which imposes run-time overhead.