Templatize spreadinterp and more cleanups #379

Job	Run time
prepare	3s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 19s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 9s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	58s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	1m 14s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 21s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 10s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	1m 4s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	52s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	12m 6s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	11m 43s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	13m 36s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	12m 16s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	11m 17s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	11m 18s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 19s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	12m 20s
Build with Pip (macos-13, 3.8, llvm, -march=native)	2m 42s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	3m 10s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	2m 37s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	2m 7s
Build with Pip (macos-13, 3.11, llvm, -march=native)	4m 5s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	2m 48s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	2m 6s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	2m 11s
	2h 7m 51s

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