dropping the inner part of the environment when an intermediate item resolved
in an outer scope.

* Remove unused argument
* Move option.t wrapping to outer functions.

Pass tydescs to parametric fns, along with (dummy, presently) retptr when needed. Can call simple parametric fn now.

module, we should look for 'b' *just* in the module 'a' and then continue
resolving b.c in the environment created by updating *with* a.

Still not 100% correct, but getting there.

find_final_def. Use the fact that the recursion is in a simpler function to
detected import loops and exit.

Change single-ident expr_ident to greedy/fat multi-ident expr_path, to handle the module-path/value-indexing distinction.

faster.

interfere with the import statements.

* If an import was unused we would never print any errors for it.
* We would keep the existing environment in scope when descending 'foo.bar'
  and would find 'bar' in the global environment if there was no 'bar' in
  'foo'.

It was not obvious how to make this implementation work when the unit size
was not also a power of two, so for now just make the buffer size a multiple
of the unit size so it can pass all the tests.