Code cleanup, make things fit in 80 columns.
Added debug info calls to CFG construction.

This is WIP (and probably temporary instrumentation anyway).  Note in
particular that most of the calls are calling `debug!` directly, but
in fact my goal is to use the `let _e = self.enter(..);` pattern
instead so that I get (1.) indentation and (2.) prints on exit.
Updates for building proof-of-concept code for testing the CFG code.
Added more debug instrumentation to the code.