[OsPath] Improve performance by using a slicable type

[hasufell]

haskell/bytestring#541 (comment)

[hasufell]

@oberblastmeister

[hasufell]

Also, my benchmarks currently show:

AFPP
    splitDirectories (posix):            OK
      4.46 μs ± 134 ns
    splitDirectories (windows):          OK
      11.8 μs ± 791 ns
FilePath
    splitDirectories (posix):            OK
      13.2 μs ± 853 ns
    splitDirectories (windows):          OK
      11.6 μs ± 495 ns