TyTy: use new subclass API

gcc/rust/ChangeLog:

	* typecheck/rust-tyty.cc (BaseType::is_unit): Refactor.
	(BaseType::satisfies_bound): Refactor.
	(BaseType::get_root): Refactor.
	(BaseType::destructure): Refactor.
	(BaseType::monomorphized_clone): Refactor.
	(BaseType::is_concrete): Refactor.
	(InferType::InferType): Refactor.
	(InferType::clone): Refactor.
	(InferType::apply_primitive_type_hint): Refactor.
	(StructFieldType::is_equal): Refactor.
	(ADTType::is_equal): Refactor.
	(handle_substitions): Refactor.
	(ADTType::handle_substitions): Refactor.
	(TupleType::TupleType): Refactor.
	(TupleType::is_equal): Refactor.
	(TupleType::handle_substitions): Refactor.

Signed-off-by: Jakub Dupak <[email protected]>