Add MP_OBJ_TYPE(o) accessor.

Should not be overused, but when needed, it's needed.
rt_is_true(): Start with testing for (expected) boolean values.
Add rt_resolve_method() which doesn't throw exception if method not found.

Unlike previosuly existing rt_load_method().
Add RT_BINARY_OP_SUBSCR_STORE and implement rt_store_subscr() in its terms.

This allows types to implement __setitem__() and thus properly support
Python sequence/dictionary protocols.