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..

Key.hs

@@ -51,6 +51,7 @@ stubKey = Key
 	, keyMtime = Nothing
 	, keyChunkSize = Nothing
 	, keyChunkNum = Nothing
+	, keySerialization = Nothing
 	}
 
 -- Gets the parent of a chunk key.
@@ -58,6 +59,7 @@ nonChunkKey :: Key -> Key
 nonChunkKey k = k
 	{ keyChunkSize = Nothing
 	, keyChunkNum = Nothing
+	, keySerialization = Nothing
 	}
 
 -- Where a chunk key is offset within its parent.
@@ -97,7 +99,9 @@ serializeKey :: Key -> String
 serializeKey = decodeBL' . serializeKey'
 
 serializeKey' :: Key -> L.ByteString
-serializeKey' = toLazyByteStringWith (safeStrategy 128 smallChunkSize) L.empty . buildKey
+serializeKey' k = case keySerialization k of
+	Nothing -> toLazyByteStringWith (safeStrategy 128 smallChunkSize) L.empty (buildKey k)
+	Just b -> L.fromStrict b
 
 {- This is a strict parser for security reasons; a key
  - can contain only 4 fields, which all consist only of numbers.
@@ -127,6 +131,7 @@ keyParser = do
 			, keyMtime = m
 			, keyChunkSize = cs
 			, keyChunkNum = cn
+			, keySerialization = Nothing
 			}
 		else fail "invalid keyName"
   where
@@ -140,7 +145,10 @@ deserializeKey :: String -> Maybe Key
 deserializeKey = deserializeKey' . encodeBS'
 
 deserializeKey' :: S.ByteString -> Maybe Key
-deserializeKey' b = eitherToMaybe $ A.parseOnly keyParser b
+deserializeKey' b = either
+	(const Nothing)
+	(\k -> Just $ k { keySerialization = Just b })
+	(A.parseOnly keyParser b)
 
 {- This splits any extension out of the keyName, returning the 
  - keyName minus extension, and the extension (including leading dot).
@@ -177,6 +185,7 @@ instance Arbitrary Key where
 		<*> ((abs . fromInteger <$>) <$> arbitrary) -- mtime cannot be negative
 		<*> ((abs <$>) <$> arbitrary) -- chunksize cannot be negative
 		<*> ((succ . abs <$>) <$> arbitrary) -- chunknum cannot be 0 or negative
+		<*> pure Nothing
 
 instance Hashable Key where
 	hashIO32 = hashIO32 . serializeKey'