Fix #580 by using a fixed-point implementation for unit conversions using integer representations #296

Job	Run time
std::format none C++20 Clang-18 Release	5m 47s
std::format none C++20 Clang-18 Debug	5m 53s
std::format none C++23 Clang-18 Release	6m 6s
std::format none C++23 Clang-18 Debug	5m 54s
std::format gsl-lite C++20 Clang-18 Release	6m 15s
std::format gsl-lite C++20 Clang-18 Debug	5m 59s
std::format gsl-lite C++23 Clang-18 Release	6m 10s
std::format gsl-lite C++23 Clang-18 Debug	6m 2s
std::format ms-gsl C++20 Clang-18 Release	6m 3s
std::format ms-gsl C++20 Clang-18 Debug	5m 58s
std::format ms-gsl C++23 Clang-18 Release	6m 3s
std::format ms-gsl C++23 Clang-18 Debug	5m 44s
fmtlib none C++20 Clang-18 Release	5m 48s
fmtlib none C++20 Clang-18 Debug	5m 43s
fmtlib none C++23 Clang-18 Release	7m 9s
fmtlib none C++23 Clang-18 Debug	5m 42s
fmtlib gsl-lite C++20 Clang-18 Release	5m 51s
fmtlib gsl-lite C++20 Clang-18 Debug	6m 4s
fmtlib gsl-lite C++23 Clang-18 Release	6m 9s
fmtlib gsl-lite C++23 Clang-18 Debug	5m 44s
fmtlib ms-gsl C++20 Clang-18 Release	5m 56s
fmtlib ms-gsl C++20 Clang-18 Debug	5m 45s
fmtlib ms-gsl C++23 Clang-18 Release	6m 2s
fmtlib ms-gsl C++23 Clang-18 Debug	6m 17s
	2h 24m 4s

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