use ByteStrings when reading annex symlinks and pointers

Now there's a ByteString used all the way from disk to Key.

The main complication in this conversion was the use of fromInternalGitPath
in several places to munge things on Windows. The things that used that
were changed to parse the ByteString using either path separator.

Also some code that had read from files to a String lazily was changed
to read a minimal strict ByteString.

Assistant/Threads/Watcher.hs

@@ -302,11 +302,11 @@ onAddUnlocked' isdirect contentchanged addassociatedfile addlink samefilestatus
 			case linktarget of
 				Nothing -> a
 				Just lt -> do
-					case fileKey $ takeFileName lt of
+					case parseLinkTarget lt of
 						Nothing -> noop
 						Just key -> liftAnnex $
 							addassociatedfile key file
-					onAddSymlink' linktarget mk isdirect file fs
+					onAddSymlink' (Just $ fromRawFilePath lt) mk isdirect file fs
 
 {- A symlink might be an arbitrary symlink, which is just added.
  - Or, if it is a git-annex symlink, ensure it points to the content