Provide a raw ByteString version of FilePath and environment APIs

The new module System.Posix.ByteString provides exactly the same API
as System.Posix, except that:

  - There is a new type: RawFilePath = ByteString

  - All functions mentioning FilePath in the System.Posix API
    use RawFilePath in the System.Posix.ByteString API

  - RawFilePaths are not subject to Unicode locale encoding and
    decoding, unlike FilePaths.  They are the exact bytes passed to and
    returned from the underlying POSIX API.

  - Similarly for functions that deal in environment
    strings (System.Posix.Env): these use untranslated ByteStrings
    in System.Posix.Environment

  - There is a new function

     System.Posix.ByteString.getArgs :: [ByteString]

    returning the raw untranslated arguments as passed to exec() when
    the program was started.