path: remove `StringPrototypeCharCodeAt` from some methods of `posix` #965

Job	Run time
Build slim tarball	1m 34s
x86_64-darwin: with shared libraries	37m 45s
aarch64-linux: with shared libraries	23m 58s
aarch64-darwin: with shared libraries	27m 8s
x86_64-linux: with shared libraries	27m 44s
	1h 58m 9s

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