TODO

* Convert all uses of `error' to have a proper symbol indicator

* Turn the testing thing into a more useful test harness: for example,
  make it run all of the tests and report errors for all failures
  instead of aborting on the first error.  Potentially use one of the
  existing testing frameworks that we have in Racket.

* Convert the (define foo (lambda (...) ...))s

* use structs instead of abusing lists & sexprs

* Mess: `lookup-variable-value' and `defined-in-env' mess: why return a
  frame instead of the value??  (Also mess in using an empty env
  essentially as a flag for raising an exception later.)

* `get-sub-frame' in the imperative interpreter keeps building bogus
  frames that are boxes, but these boxes are not intended to be changed.