Add type-specific optimizations for popcount #56

Job	Run time
build (ninja-gcc, 17, Debug)	48s
build (ninja-gcc, 17, Release)	57s
build (ninja-gcc, 20, Debug)	52s
build (ninja-gcc, 20, Release)	53s
build (ninja-clang, 17, Debug)	53s
build (ninja-clang, 17, Release)	1m 3s
build (ninja-clang, 20, Debug)	57s
build (ninja-clang, 20, Release)	1m 5s
build (ninja-clang-libcpp, 17, Debug)	59s
build (ninja-clang-libcpp, 17, Release)	1m 5s
build (ninja-clang-libcpp, 20, Debug)	58s
build (ninja-clang-libcpp, 20, Release)	1m 2s
test (ninja-gcc, 17, Debug)	11s
test (ninja-gcc, 17, Release)	9s
test (ninja-gcc, 20, Debug)	13s
test (ninja-gcc, 20, Release)	12s
test (ninja-clang, 17, Debug)	9s
test (ninja-clang, 17, Release)	10s
test (ninja-clang, 20, Debug)	9s
test (ninja-clang, 20, Release)	12s
test (ninja-clang-libcpp, 17, Debug)	11s
test (ninja-clang-libcpp, 17, Release)	10s
test (ninja-clang-libcpp, 20, Debug)	15s
test (ninja-clang-libcpp, 20, Release)	14s
	13m 47s

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