Revert "Introduce new superclass for (almost) all classes"

This reverts commit 8442a48d67fa112a190c12ccdb38bb57f0427773.

Conflicts:
	pycket/prims.py
Revert "Make Cont translatable"

This reverts commit 4e4bf9dc5befcf0c481e46d6f054d9099ae6e038.

Conflicts:
	pycket/prims.py
Revise variable handling to use consistent types.

* assign_convert and mutated_vars work with Vars and variable_sets.
* free_vars works with W_Symbols and dicts.