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.

Command/Migrate.hs

@@ -86,7 +86,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey (fastMigrate oldbacken
 		content <- calcRepo $ gitAnnexLocation oldkey
 		let source = KeySource
 			{ keyFilename = fromRawFilePath file
-			, contentLocation = content
+			, contentLocation = fromRawFilePath  content
 			, inodeCache = Nothing
 			}
 		v <- genKey source nullMeterUpdate (Just newbackend)