Numeric binary operator support for old style class

[Daetalus]

No description provided.

[kmod]

I'm not sure I understand the role of these functions -- they seem named after the typeobject.c functions, but
behave a bit more like instance_pow. Regardless, maybe in this situation we should just copy instance_pow and instance_ipow from CPython? Your slot_nb_power seems reasonable (though a bit hard to verify all the corner cases); I think inplace_power is missing the fallback to __pow__ and __rpow__.

[Daetalus]

Is that ok to copy these code? Or use a "Pyston way" function? Such as generic_binary_op could replaced by _instanceBinary function. But for other functions, such as half_binop and do_pow, it contains logic. So either rewrite it ( like the last version of commit), or call the CPython version.

[kmod]

I think copying the cpython code is the right approach here -- we can go back and make changes to it if we need to, but basically it just saves so many headaches to use code that we know is right :) We might want at some point to reduce the duplication by having our functions call the cpython ones you just copied.