path: remove `StringPrototypeCharCodeAt` from some methods of `posix`
confidence improvement accuracy (*) (**) (***)
path/normalize-posix.js n=100000 path='.' *** 7.77 % ±1.84% ±2.44% ±3.18%
path/normalize-posix.js n=100000 path='' 3.09 % ±4.67% ±6.27% ±8.28%
path/normalize-posix.js n=100000 path='/../' * 2.57 % ±1.97% ±2.62% ±3.41%
path/normalize-posix.js n=100000 path='/foo' * 2.54 % ±1.94% ±2.57% ±3.35%
path/normalize-posix.js n=100000 path='/foo/bar' 1.30 % ±1.99% ±2.65% ±3.45%
path/normalize-posix.js n=100000 path='/foo/bar//baz/asdf/quux/..' 0.70 % ±1.78% ±2.37% ±3.08%
path/parse-posix.js n=100000 path='' -0.82 % ±2.25% ±2.99% ±3.90%
path/parse-posix.js n=100000 path='/' ** 2.35 % ±1.62% ±2.16% ±2.81%
path/parse-posix.js n=100000 path='/foo' * 2.01 % ±1.95% ±2.59% ±3.37%
path/parse-posix.js n=100000 path='/foo/bar.baz' ** 1.66 % ±1.22% ±1.62% ±2.11%
path/parse-posix.js n=100000 path='/foo/bar/baz/asdf/.quux' 0.74 % ±1.46% ±1.95% ±2.53%
path/parse-posix.js n=100000 path='foo/.bar.baz' -0.93 % ±1.32% ±1.76% ±2.29%
path/parse-posix.js n=100000 path='foo/bar' 1.32 % ±1.36% ±1.82% ±2.37%
path/resolve-posix.js n=100000 paths='' 1.57 % ±2.29% ±3.06% ±4.04%
path/resolve-posix.js n=100000 paths='|' ** 1.10 % ±0.78% ±1.03% ±1.34%
path/resolve-posix.js n=100000 paths='a/b/c/|../../..' 0.47 % ±0.71% ±0.95% ±1.23%
path/resolve-posix.js n=100000 paths='foo/bar|/tmp/file/|..|a/../subfile' -0.00 % ±1.20% ±1.60% ±2.08%