Reintroduce constexpr fmt::formatted_size for C++20 #807

Job	Run time
mingw (mingw64)	3m 42s
build (windows-2019, Win32, v142, 17, Debug)	2m 14s
mingw (ucrt64)	3m 53s
build (windows-2019, Win32, v142, 17, Release)	2m 59s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 8s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	1m 36s
build (windows-2019, x64, v140, 14, Debug)	1m 29s
build (windows-2019, x64, v140, 14, Release)	1m 36s
build (windows-2019, x64, v141, 17, Debug)	1m 17s
build (windows-2019, x64, v141, 17, Release)	1m 25s
build (windows-2019, x64, v142, 17, Debug)	54s
build (windows-2019, x64, v142, 17, Release)	1m 17s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 18s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	1m 19s
build (windows-2019, x64, v142, 20, Debug)	57s
build (windows-2019, x64, v142, 20, Release)	1m 7s
build (windows-2022, x64, v143, Debug, 20)	47s
	28m 58s

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