General cleanup in preparation for a bugfix: removed unused code, useless

General cleanup in preparation for a bugfix:  removed unused code, useless
declarations, added some comments where I had to think too hard to
understand what was happening, and changed the primary internal get/set
functions to assert they're passed objects of the correct type instead of
doing runtime tests for that (it's an internal error that "should never
happen", so it's good enough to check it only in the debug build).