Use more maths to describe the fixed-point functions #923

Job	Run time
macos-static (macos-12)	2m 59s
unix (ubuntu-20.04, g++, make)	5m 8s
windows-mingw-build (32)	1m 20s
unix (ubuntu-20.04, g++, cmake)	4m 52s
windows (32, windows-2019)	5m 4s
windows-mingw-build (64)	51s
unix (ubuntu-20.04, clang++, make)	5m 21s
windows (32, windows-2022)	5m 46s
unix (ubuntu-20.04, clang++, cmake)	5m 23s
windows (64, windows-2019)	5m 1s
windows (64, windows-2022)	6m 28s
unix (ubuntu-22.04, g++, make)	4m 51s
unix (ubuntu-22.04, g++, cmake)	5m 25s
unix (ubuntu-22.04, clang++, make)	3m 41s
unix (ubuntu-22.04, clang++, cmake)	6m 20s
unix (macos-12, clang++, make)	11m 43s
unix (macos-12, clang++, cmake)	9m 26s
windows-mingw-testing (windows-2019, 32)	4m 7s
windows-mingw-testing (windows-2019, 64)	4m 1s
windows-mingw-testing (windows-2022, 32)	4m 36s
windows-mingw-testing (windows-2022, 64)	3m 53s
	1h 46m 16s

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