Polytypes module folded into types module, with all the actual type
information done when you load the library module. Some definitions can
be inferred from their body expression, others need to be
CombinatorJoyType wrapped. Still to do: loop.
If type vars get into the espression you have to keep them in sync with
the unification or you can lose information.
Some combinators can put symbols on the expression, you have to convert
those to type checkers or, as a hack, just look them up and run them.
This lets definitions work(-ish), ...
So they can notice if they're given a stack that doesn't match what
they're expecting.
This seems to work, but I realized that type variables in the pending
expression need to be update()'d too. hmm...
I'm backfilling tests to cover the functionality that I developed
incrementally in the Jupyter notebook and nail it down with concrete
examples. No doubt I'll uncover some bugs.
Simon Forman