remove unused backend machinery

The only remaining vestiage of backends is different types of keys. These
are still called "backends", mostly to avoid needing to change user interface
and configuration. But everything to do with storing keys in different
backends was gone; instead different types of remotes are used.

In the refactoring, lots of code was moved out of odd corners like
Backend.File, to closer to where it's used, like Command.Drop and
Command.Fsck. Quite a lot of dead code was removed. Several data structures
became simpler, which may result in better runtime efficiency. There should
be no user-visible changes.

Types/Backend.hs

@@ -16,22 +16,8 @@ data Backend a = Backend {
 	name :: String,
 	-- converts a filename to a key
 	getKey :: FilePath -> a (Maybe Key),
-	-- stores a file's contents to a key
-	storeFileKey :: FilePath -> Key -> a Bool,
-	-- retrieves a key's contents to a file
-	retrieveKeyFile :: Key -> FilePath -> a Bool,
-	-- removes a key, optionally checking that enough copies are stored
-	-- elsewhere
-	removeKey :: Key -> Maybe Int -> a Bool,
-	-- checks if a backend is storing the content of a key
-	hasKey :: Key -> a Bool,
 	-- called during fsck to check a key
-	-- (second parameter may be the filename associated with it)
-	-- (third parameter may be the number of copies that there should
-	-- be of the key)
-	fsckKey :: Key -> Maybe FilePath -> Maybe Int -> a Bool,
-	-- Is a newer repesentation possible for a key?
-	upgradableKey :: Key -> a Bool
+	fsckKey :: Key -> a Bool
 }
 
 instance Show (Backend a) where