RawFilePath will need to support Windows too

Of course, readSymbolicLink always fails on Windows, but now it's ready
for other things that don't fail there.

Utility/RawFilePath.hs

@@ -1,4 +1,11 @@
 {- Portability shim around System.Posix.Files.ByteString
+ -
+ - On unix, this makes syscalls using RawFilesPaths as efficiently as
+ - possible.
+ -
+ - On Windows, filenames are in unicode, so RawFilePaths have to be
+ - decoded. So this library will work, but less efficiently than using
+ - FilePath would.
  -
  - Copyright 2019 Joey Hess <id@joeyh.name>
  -
@@ -17,12 +24,9 @@ import System.Posix.Files.ByteString
 import System.Posix.ByteString.FilePath
 #else
 import qualified Data.ByteString as B
-import System.IO.Error
-
-type RawFilePath = B.ByteString
+import qualified System.PosixCompat as P
+import Utility.FileSystemEncoding
 
 readSymbolicLink :: RawFilePath -> IO RawFilePath
-readSymbolicLink _ = ioError $ mkIOError illegalOperationErrorType x Nothing Nothing
-  where
-	x = "Utility.RawFilePath.readSymbolicLink: not supported"
+readSymbolicLink f = toRawFilePath <$> P.readSymbolicLink (fromRawFilePath f)
 #endif