cache the serialization of a Key

This will speed up the common case where a Key is deserialized from
disk, but is then serialized to build eg, the path to the annex object.

It means that every place a Key has any of its fields changed, the cache
has to be dropped. I've grepped and found them all. But, it would be
better to avoid that gotcha somehow..

Remote/Helper/Chunked.hs

@@ -68,7 +68,10 @@ chunkKeyStream :: Key -> ChunkSize -> ChunkKeyStream
 chunkKeyStream basek chunksize = ChunkKeyStream $ map mk [1..]
   where
 	mk chunknum = sizedk { keyChunkNum = Just chunknum }
-	sizedk = basek { keyChunkSize = Just (toInteger chunksize) }
+	sizedk = basek
+		{ keyChunkSize = Just (toInteger chunksize)
+		, keySerialization = Nothing
+		}
 
 nextChunkKeyStream :: ChunkKeyStream -> (Key, ChunkKeyStream)
 nextChunkKeyStream (ChunkKeyStream (k:l)) = (k, ChunkKeyStream l)