Cython closures not bound as methods

[robertwb]

This is especially relevant to using closures to decorate methods.

sage: class A:
....:     foo = lambda *x: x
....:     
sage: A.foo
<unbound method A.<lambda>>
sage: A().foo
<bound method A.<lambda> of <__main__.A instance at 0xacafee0>>
sage: A().foo()
(<__main__.A instance at 0xacd8080>,)

Rather than using the method binding trick ourselves, we need to make closures into a class with __call__, __get__, and __set__ methods.

[robertwb]

See also #494

[robertwb]

Tests at  http://hg.cython.org/cython-closures/rev/ea90bf6a4c3a now that the #494 framework is in place.