Add abc::_raw_vector specialization fork based on whether T is copyable
Improve semantics of abc::_raw_vextr_*_impl::assign_copy() and assign_move_dynamic_or_copy()
Rename abc::_raw_vextr_*_impl::assign_move_dynamic_or_copy() to assign_move_dynamic_or_move_items()

The new name reflects its effects more accurately.