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.