libstdc++: Add fast path for std::format("{}", x) [PR110801]

This optimizes the simple case of formatting a single string, integer
or bool, with no format-specifier (so no padding, alignment, alternate
form etc.)

libstdc++-v3/ChangeLog:

	PR libstdc++/110801
	* include/std/format (_Sink_iter::_M_reserve): New member
	function.
	(_Sink::_Reservation): New nested class.
	(_Sink::_M_reserve, _Sink::_M_bump): New virtual functions.
	(_Seq_sink::_M_reserve, _Seq_sink::_M_bump): New virtual
	overrides.
	(_Iter_sink<O, ContigIter>::_M_reserve): Likewise.
	(__do_vformat_to): Use new functions to optimize "{}" case.