Templatize spreadinterp and more cleanups #382

Job	Run time
prepare	2s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 11s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 10s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	57s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	56s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 12s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 19s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	52s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	53s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 50s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	10m 48s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	12m 11s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	11m 23s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	12m 35s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	12m 14s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	11m 35s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	12m 32s
Build with Pip (macos-13, 3.8, llvm, -march=native)	2m 49s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	2m 54s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	1m 40s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	2m 5s
Build with Pip (macos-13, 3.11, llvm, -march=native)	4m 0s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	2m 51s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	2m 15s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	1m 40s
	2h 3m 54s

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