Implement AugAssign

I just got a chance to test this locally, looks like it works as expected.

This isn't the perfect, mostly the last three lines I think should be combinable into one, but `evaluate_expr` doesn't result in a storable object, and `evaluate_expr_store` doesn't result in a modifiable one, but this works fine for now.

This should close #56.

I also cleared up some code at the end of execcode_boolop. I'm pretty sure only the two operations exist, so the code should be dead anyway, but in case something weird does come up, it won't try to find a nonexistent opfunc.