descriptor support for __get__ and __set__

Descriptor support for `__get__` and `__set__` in the `getattr`, `getclsattr`, and `callattr` paths.

- The `getattr` path follows the protocol described at https://docs.python.org/2/howto/descriptor.html. There are two cases: the object is an instance and the instance is a class.
  - For instances, It looks up attribute in the object's dictionary and the attribute from the class using `typeLookup`. In the latter, it looks for the `__get__` method, but if the attribute exists in _both_ it also checks that `__set__` exists.
  - For classes, it's simpler; it just looks if `__get__` exists in the looked-up item.
- The `getclsattr` path is sort of a mix - it looks up the attribute just in the class but will call `__get__` with the object rather than `None`.
- For `callattr`, the code for looking up the value seemed completely wrong (e.g., it was doing custom lookups through `__getattr__` before looking up the class object) so I scrapped that part. I figured this should basically share all logic with `getattr` or `getclsattr` as appropriate - except that it shouldn't wrap functions in instancemethods. So I refactored `getattr_internal` out into `getattr_internal_general` which takes an additional flag which species that it should not wrap functions, and `callattr` just calls that. (So `callattr` still does binding if necessary.) Similarly, I factored `clsattr_internal` into `clsattr_internal_general`.

One thing of note is that it currently doesn't rewrite the path where `callattr` calls `__get__`, since that involves _two_ function calls: one to `__get__` and another to the function that `__get__` returns. Of course, this path is basically just a getattr and a call, each of which could be written by itself, so this clearly isn't a fundamental limitation.

I got rid of `_handleClsAttr` which already had special-case descriptor logic for functions and member descriptors. Those cases are still handled specially though, just in getattr and getclsattr with the other descriptor logic.

Finally, for `setattr`, I check if `__set__` exists and if so call that.
Oh I just meant that right now instancemethods will get printed as "bound instancemethod object" even if they're unbound, and maybe they should just say "unbound method" if they're not bound.  But yes, they should also be updated to give the same format as CPython.