Implement WeakIdentityDict for prop values storage

Suppose there is a class A:

    class A(object):
        id = norm.orm.props.Int()

        def __hash__(self):
            return hash(self.id)

Currently calling hash(A()) leads to infinite recursion,
because getting self.id leads to insertion of self into a
WeakKeyDictionary, which, in turn, calls self.__hash__().

This patch implements an alternate id-based dictionary class
for property storage. This WeakIdentityDict never calls any
methods on its keys, and is interface-identical to the
weakref.WeakKeyDictionary.