Reintroduce constexpr fmt::formatted_size for C++20 #817

Job	Run time
build (windows-2019, Win32, v142, 17, Debug)	1m 26s
build (windows-2019, Win32, v142, 17, Release)	2m 20s
mingw (mingw64)	4m 2s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	2m 16s
mingw (ucrt64)	4m 14s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 56s
build (windows-2019, x64, v140, 14, Debug)	2m 28s
build (windows-2019, x64, v140, 14, Release)	2m 28s
build (windows-2019, x64, v141, 17, Debug)	1m 24s
build (windows-2019, x64, v141, 17, Release)	2m 21s
build (windows-2019, x64, v142, 17, Debug)	1m 28s
build (windows-2019, x64, v142, 17, Release)	2m 19s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 58s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 8s
build (windows-2019, x64, v142, 20, Debug)	1m 58s
build (windows-2019, x64, v142, 20, Release)	1m 53s
build (windows-2022, x64, v143, Debug, 20)	1m 59s
	39m 38s

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