Add support for FilePath

[zouppen]

In my application I have difficulty to do reliable conversion of FilePath -> RawFilePath because of UTF-8 encoding of my path names in my operating system and if the file contains non-ASCII characters such as ä. Therefore it would be a good idea to add an alternative version of addWatch supporting regular FilePath. Without removing RawFilePath support, of course.

I know that RawFilePath is more performant, but in this case I don't need the performance but I'd like to see it not to break.

In case there is off-the-shelf solution for FilePath -> RawFilePath conversion which works on all file system encodings, please let me know and perhaps add it to README because some others might struggle with that. :-)

[thomasjm]

FWIW, the fsnotify package does it like this:

toRawFilePath :: FilePath -> IO BS.ByteString
toRawFilePath fp = do
  enc <- getFileSystemEncoding
  F.withCString enc fp BS.packCString

fromRawFilePath :: BS.ByteString -> IO FilePath
fromRawFilePath bs = do
  enc <- getFileSystemEncoding
  BS.useAsCString bs (F.peekCString enc)