Clarify that we're only doing PY2 classes, and add tests

In PY3, we're falling back to the builtin implementation by relying on __build_class__.

Add tests that show that something is going wrong with attribute lookup.
expose a very bad scoping bug
Fix scoping bug.

This causes the PY2 class inheiritance tests to start failing - they were only working because of this scoping bug, I think.
split up generator tests
Fix generator implementation

I've come to believe that the right implementation of the VM has one data stack per frame, not one data stack for the VM.  This is consistent with halting and resuming frames using generators.  The bug appeared to be around nested generators, but in fact it should arise with any generator resuming a frame if the stack has been modified in the meantime.
small bug in closure naming
Another failing test of bad attribute lookup
Tests of descriptors
Rip out the object system :(
Merged pull request 10