Rename primesApprox() to primePiApprox() #52

Job	Run time
linux_gcc_sanitizers	2m 20s
linux_gcc (ubuntu-latest, Debug)	4m 35s
linux_clang (ubuntu-latest, Debug)	3m 48s
linux_gcc (ubuntu-latest, Release)	2m 7s
linux_clang (ubuntu-latest, Release)	1m 17s
linux_gcc_valgrind	1m 15s
macos_clang (macos-latest, Debug)	5m 47s
macos_clang (macos-latest, Release)	1m 31s
windows_mingw64 (windows-latest, Debug)	5m 24s
windows_msvc2022	2m 6s
windows_mingw64 (windows-latest, Release)	2m 53s
	33m 3s

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