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..

Types/Key.hs

@@ -23,6 +23,7 @@ data Key = Key
 	, keyMtime :: Maybe EpochTime
 	, keyChunkSize :: Maybe Integer
 	, keyChunkNum :: Maybe Integer
+	, keySerialization :: Maybe S.ByteString -- ^ cached serialization
 	} deriving (Eq, Ord, Read, Show)
 
 {- A filename may be associated with a Key. -}