- Reverse the code for workers and wrappers for nullary constructors.
  For some reason it was the wrong way around, but the effects were
  harmless since they both evaluate to the same thing.

- When passing a nullary constructor as an argument, we should pass
  the name of the worker rather than the wrapper.  Again, this is
  mostly harmless, but it enables some small simplification in
  pushAtom.

- Rearrange/cleanup pushAtom.