Change `static_cast` to `unsafe_narrow` #48

Job	Run time
Run on ubuntu-latest using g++-10	2m 59s
Run on ubuntu-latest using g++-13	3m 10s
Run on ubuntu-20.04 using clang++-12	2m 45s
Run on macos-13 using clang++	3m 30s
Run on windows-latest using cl.exe	2m 43s
	15m 7s

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