Results of today's hacking:

  - We can now execute expressions from the GHCi prompt.  However,
    a problem with the typechecker environment means that identifiers
    from outside the current module aren't resolved :-(

  - loading up a multi-module program in the interpreter seems to
    work.  Interpreting is kinda slow (ok, very slow), but I'm hoping
    it'll get better when I compile the interpreter w/ optimisation.

  - :set sort of works - you can do ":set -dshow-passes", for example

  - lots of bugfixes, etc.