Python classes defining __slots__ should automatically compile to extension classes

[robertwb]

In pure Python mode, when a Python class with 0-1 known base classes defines a constant __slots__ attribute, it makes sense to compile it into an extension class automatically, and make the defined slots object attributes.

Migrated from http://trac.cython.org/ticket/433

[scoder]

Can be implemented as part of the AlignFunctionDefinitions transform, which essentially does the same thing for Python class type overrides found in .pxd files.

[jdemeyer]

Because of the dynamic nature of Python classes, doing this unconditionally might break Python compatibilty. I would rather to it as opt-in: some directive to do it for all classes for __slots__ and for every class which is already declared as cdef class.