Get rid of "wrapper_descriptors" and "method-wrapper"s #714

>>> type(int.__add__)
<class 'wrapper_descriptor'>
>>> type((0).__add__)
<class 'method-wrapper'>

Compare to

>>> type(int.bit_count)
<class 'method_descriptor'>
>>> type((0).bit_count)
<class 'builtin_function_or_method'>

Wrapper descriptors and method wrappers are artifacts of the implementation. Ideally they would not exist and we would get

>>> type(int.__add__)
<class 'method_descriptor'>
>>> type((0).__add__)
<class 'builtin_function_or_method'>

We would like these wrapper descriptors to be method descriptors in order to simplify optimization and generally reduce the number of special cases.

There are two obstacles to doing this.

1. These two types are exposed in the types module, so we cannot remove them altogether. Maybe we could make types.WrapperDescriptorType a subclass of method descriptor to simplify things, or just make types.WrapperDescriptorType an alias?
2. There is no way to efficiently wrap a binary C function in a method descriptor. This is easily fixed by adding a METH_OO flag for functions taking two arguments enabling effective specialization of these functions