Register BTree/Bucket as MutableMapping and Set/TreeSet as MutableSet, add missing methods/operators

[jamadden]

Add tests to verify they implement the required methods. Turns out they didn't, so implement them.
This means popitem for the mappings, and pop/discard/isdisjoint plus a swath of operators for the sets.

Do this in both C and Python. The implementations are probably on the naive side, in that they work with generic iterators and don't try to do any complex structural merging, but that makes them easier to understand and hopefully be correct. Performance shouldn't be too bad, nothing worse than O(N*logM) (roughly). The __iand__ method requires extra storage space of up to O(N), and __xor__ requires storage of O(N*M).

Fixes #121

This was a lot bigger than I expected.

[icemac]

LGTM for the Python code. I cannot judge the C code.

[jamadden]

LGTM for the Python code. I cannot judge the C code.

I firmly admit to not being a C API expert. I'm at best a journeyman, and I am responsible for at least one true memory leak in zope.interface. That leak involved garbage collection, not regular methods.

Here, I've done my best to keep the C implementations simple, and they don't involve type-level GC changes, so I hope that's a point in favor of this PR :) Also, these are mostly new methods and operators, so even if there are some memory leaks, they won't be sudden and widespread — hopefully people can help report them.

I think I can mostly rule out obvious memory misuse errors, as they would result in segfaults; I know, I experienced them several times during development.

As always, more eyeballs are welcome, so I will wait at least several hours before merging (it's after my bedtime in my timezone anyway).

Thank you!