objtype: Implement basic framework for subclassing native types.

This implements checking of base types, allocation and basic initialization,
and optimized support for special method lookups. Other features are not yet
supported.
tests: Add basic tests for subclassing native types and using special methods.

Even of these, some features do not yet work as expected.
objtype: Add support for looking up non-method attrs in native base class.
tests: Add test for accessing attribute of inherited native type.
objtype: Support calling normal methods inherited from native base class.

Biggest part of this support is refactoring mp_obj_class_lookup() to return
standard "bound member" pair (mp_obj_t[2]). Actual support of inherited
native methods is 3 lines then. Some inherited features may be not supported
yet (e.g. native class methods, native properties, etc., etc.). There may
be opportunities for further optimization too.
tests: Add test for calling inherited native method on subclass.
Merge pull request #528 from pfalcon/native-subclass1

Initial support for subclassing native types