std::format parsing callbacks and machinery

[barcharcraz]

• adds _Parse_format_string
• adds _Parse_replacement_field
• adds _Specs_setter and _Basic_format_specs
• adds _Specs_checker, and _Numeric_specs_checker
• adds _Format_handler (note that _On_format_specs is incomplete)
• adds vformat_to definition
• adds basic_format_args::get

in total this is enough to get vformat_to instantiations to compile, although we can't run them without a few more basic_format_arg and basic_format_args related things.

Note that this vformat_to is the C++ 20 version of vformat_to, NOT the proposed C++23 version (they have different ABIs)

Also note that this PR contains merge commits, they should go away if it's squashed, however.

[miscco]

I think that if _Begin == _End we will always end up in the throw error case.

At least for me It would be clearer if we would write it like that

Suggested change

	_CharT _Ch = _CharT{};
	if (_Begin != _End) {
	_Ch = *_Begin;
	}
	if (_Begin == _End) {
	throw format_error("Missing '}' in format string.");
	}
	const _CharT _Ch = *_Begin;

[CaseyCarter]

Feel free to merge as-is and make changes in a later PR.