add routes to pause/start/cancel transfers

This commit includes a paydown on technical debt incurred two years ago,
when I didn't know that it was bad to make custom Read and Show instances
for types. As the routes need Read and Show for Transfer, which includes a
Key, and deriving my own Read instance of key was not practical,
I had to finally clean that up.

So the compact Key read and show functions are now file2key and key2file,
and Read and Show are now derived instances.

Changed all code that used the old instances, compiler checked.
(There were a few places, particularly in Command.Unused, and the test
suite where the Show instance continue to be used for legitimate
comparisons; ie show key_x == show key_y (though really in a bloom filter))

Types/Key.hs

@@ -10,9 +10,10 @@
 module Types.Key (
 	Key(..),
 	stubKey,
-	readKey,
+	key2file,
+	file2key,
 
-	prop_idempotent_key_read_show
+	prop_idempotent_key_encode
 ) where
 
 import System.Posix.Types
@@ -26,7 +27,7 @@ data Key = Key {
 	keyBackendName :: String,
 	keySize :: Maybe Integer,
 	keyMtime :: Maybe EpochTime
-} deriving (Eq, Ord)
+} deriving (Eq, Ord, Read, Show)
 
 stubKey :: Key
 stubKey = Key {
@@ -39,21 +40,21 @@ stubKey = Key {
 fieldSep :: Char
 fieldSep = '-'
 
-{- Keys show as strings that are suitable for use as filenames.
+{- Converts a key to a strings that are suitable for use as a filename.
  - The name field is always shown last, separated by doubled fieldSeps,
  - and is the only field allowed to contain the fieldSep. -}
-instance Show Key where
-	show Key { keyBackendName = b, keySize = s, keyMtime = m, keyName = n } =
-		b +++ ('s' ?: s) +++ ('m' ?: m) +++ (fieldSep : n)
-		where
-			"" +++ y = y
-			x +++ "" = x
-			x +++ y = x ++ fieldSep:y
-			c ?: (Just v) = c : show v
-			_ ?: _ = ""
+key2file :: Key -> FilePath
+key2file Key { keyBackendName = b, keySize = s, keyMtime = m, keyName = n } =
+	b +++ ('s' ?: s) +++ ('m' ?: m) +++ (fieldSep : n)
+	where
+		"" +++ y = y
+		x +++ "" = x
+		x +++ y = x ++ fieldSep:y
+		c ?: (Just v) = c : show v
+		_ ?: _ = ""
 
-readKey :: String -> Maybe Key
-readKey s = if key == Just stubKey then Nothing else key
+file2key :: FilePath -> Maybe Key
+file2key s = if key == Just stubKey then Nothing else key
 	where
 		key = startbackend stubKey s
 
@@ -73,5 +74,5 @@ readKey s = if key == Just stubKey then Nothing else key
 		addfield 'm' k v = Just k { keyMtime = readish v }
 		addfield _ _ _ = Nothing
 
-prop_idempotent_key_read_show :: Key -> Bool
-prop_idempotent_key_read_show k = Just k == (readKey . show) k
+prop_idempotent_key_encode :: Key -> Bool
+prop_idempotent_key_encode k = Just k == (file2key . key2file) k