more OsPath conversion

keyFile has a nice improvement; since a Key is a ShortByteString, it can
be converted to an OsPath without needing the copy that was done before.

Unfortunately, fileKey has to convert from a ShortByteString to a
ByteString in order to use attoparsec, and then the results get
converted back to an OsPath, so there are now 2 copies.
Maybe attoparsec will eventually get a ShortByteString API,
see https://github.com/haskell/attoparsec/issues/225

Sponsored-by: Joshua Antonishen

Key.hs

@@ -18,6 +18,7 @@ module Key (
 	keyParser,
 	serializeKey,
 	serializeKey',
+	serializeKey'',
 	deserializeKey,
 	deserializeKey',
 	nonChunkKey,
@@ -31,7 +32,7 @@ module Key (
 
 import qualified Data.Text as T
 import qualified Data.ByteString as S
-import qualified Data.ByteString.Short as S (toShort, fromShort)
+import Data.ByteString.Short (ShortByteString, toShort, fromShort)
 import qualified Data.Attoparsec.ByteString as A
 
 import Common
@@ -63,7 +64,10 @@ serializeKey :: Key -> String
 serializeKey = decodeBS . serializeKey'
 
 serializeKey' :: Key -> S.ByteString
-serializeKey' = S.fromShort . keySerialization
+serializeKey' = fromShort . keySerialization
+
+serializeKey'' :: Key -> ShortByteString
+serializeKey'' = keySerialization
 
 deserializeKey :: String -> Maybe Key
 deserializeKey = deserializeKey' . encodeBS
@@ -73,7 +77,7 @@ deserializeKey' = eitherToMaybe . A.parseOnly keyParser
 
 instance Arbitrary KeyData where
 	arbitrary = Key
-		<$> (S.toShort . encodeBS <$> (listOf1 $ elements $ ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ "-_\r\n \t"))
+		<$> (toShort . encodeBS <$> (listOf1 $ elements $ ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ "-_\r\n \t"))
 		<*> (parseKeyVariety . encodeBS <$> (listOf1 $ elements ['A'..'Z'])) -- BACKEND
 		<*> ((abs <$>) <$> arbitrary) -- size cannot be negative
 		<*> ((abs . fromInteger <$>) <$> arbitrary) -- mtime cannot be negative