Make weakref proxies delegate comparison operators

[gfmcknight]

Add slots for all comparison operators which delegate to the object
being proxied.

Resolves #697

[slozier]

Looks good to me. Not too familiar with this part of the codebase so hopefully we're not breaking anything!

[slozier]

Neat, didn't know about SlotField.

[slozier]

I finally got around to running some tests, there's a minor issue when creating a proxy of a type that doesn't have rich comparison operators. However I think it should be resolved by #709 since it'll add the operators to the object type. Will hold off on merging this PR until the other one is ready. In case you're interested here's the failure I ran into:

>>> import _weakref
>>> class A: pass
...
>>> a = A()
>>> p = _weakref.proxy(a)
>>> p.__lt__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: Operation is not valid due to the current state of the object.

[slozier]

Thanks for the PR! Will merge it in now even though #709 isn't ready.