Templatize spreadinterp and more cleanups #408

Job	Run time
prepare	6s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 20s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 22s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	56s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	1m 0s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 17s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 12s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	1m 2s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	1m 1s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 55s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	12m 9s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	11m 55s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	12m 27s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	13m 3s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	11m 57s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 1s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	12m 18s
Build with Pip (macos-13, 3.8, llvm, -march=native)	2m 43s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	4m 45s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	1m 52s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	2m 17s
Build with Pip (macos-13, 3.11, llvm, -march=native)	2m 50s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	4m 17s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	1m 57s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	2m 10s
	2h 9m 52s

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