This patch is the start of a series to improve the speed of std::format,
std::format_to, std::format_to_n, and std::formatted_size.
This is mostly achieved by changing the __output_buffer class. This new
__output_buffer class also makes it easier to implement buffering for
P3107R5 "Permit an efficient implementation of std::print"
This is an intermediate and fairly mechanical step towards unifying the
benchmarks with the rest of the test suite. Moving this around requires
a few changes, notably making sure we don't throw a wrench into the
discovery process of the normal test suite. This won't be a problem
anymore once benchmarks are taken into account by the test setup out of
the box.
