checker,cgen: add T.key_type, typeof(expr).key_type, T.value_type, typeof(expr).value_type, T.element_type, typeof(expr).element_type for getting Map[K]V and []T types

checker,cgen: add T.key_type, typeof(expr).key_type, T.value_type, typeof(expr).value_type, T.element_type, typeof(expr).element_type for getting Map[K]V and []T types #3550