use filepath-bytestring for annex object manipulations

git-annex find is now RawFilePath end to end, no string conversions.
So is git-annex get when it does not need to get anything.
So this is a major milestone on optimisation.

Benchmarks indicate around 30% speedup in both commands.

Probably many other performance improvements. All or nearly all places
where a file is statted use RawFilePath now.

Remote/WebDAV/DavLocation.hs

@@ -39,9 +39,9 @@ keyDir :: Key -> DavLocation
 keyDir k = addTrailingPathSeparator $ hashdir </> keyFile k
   where
 #ifndef mingw32_HOST_OS
-	hashdir = hashDirLower def k
+	hashdir = fromRawFilePath $ hashDirLower def k
 #else
-	hashdir = replace "\\" "/" (hashDirLower def k)
+	hashdir = replace "\\" "/" (fromRawFilePath $ hashDirLower def k)
 #endif
 
 keyLocation :: Key -> DavLocation