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.