Is `container_appender` still needed?

[saki7]

container_appender (introduced by me) is a implementation-detail facility for flattening the container attribute types. It acts like a placeholder type to preserve the fact that "this attribute should be treated like a container, NOT a container of a container" concept; in pseudo-code: vector<int>{} >> vector<int>{} should be treated as vector<int>{}, i.e., NOT vector<vector<int>>{}.

The direct reason why I introduced container_appender is because I found that some internal code simply cannot be compiled due to complex attribute type handling involved in parse_sequence and parse_into_container. As a downside, it requires some overhead for move-appending the elements:

x4/include/iris/x4/rule.hpp

Lines 451 to 456 in e811b03

	if constexpr (is_ttp_specialization_of_v<std::remove_const_t<Exposed>, container_appender>) {
	traits::append(
	exposed_attr.container,
	std::make_move_iterator(traits::begin(rule_attr)),
	std::make_move_iterator(traits::end(rule_attr))
	);

If we keep using container_appender, the above workaround theoretically cannot be eliminated because the rule parser's attribute cannot accept container_appender<Container> when Container is some user-provided type that is used by the rule which is intended to be instantiated in a different translation unit (cf. IRIS_X4_INSTANTIATE).

After #42 is merged, we should reinvestigate whether container_appender is still needed in the codebase.