allow using Aeson for streaming JSON output

Keeping Text.JSON use for now, because it seems a better fit for most of
the commands, which don't use very structured JSON objects, but just output
whatever fields suites them. But this lets Aeson be used when a more
structured data type is available to serialize to JSON.

Messages/JSON.hs

@@ -1,6 +1,6 @@
-{- git-annex JSON output
+{- git-annex command-line JSON output and input
  -
- - Copyright 2011 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2016 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -21,7 +21,7 @@ import Types.Key
 import Data.Maybe
 
 start :: String -> Maybe FilePath -> Maybe Key -> IO ()
-start command file key = putStr $ Stream.start $ catMaybes
+start command file key = putStr $ Stream.start $ Stream.JSONObject $ catMaybes
 	[ part "command" (Just command)
 	, part "file" file
 	, part "key" (fmap key2file key)
@@ -31,15 +31,15 @@ start command file key = putStr $ Stream.start $ catMaybes
 	part l (Just v) = Just (l, v)
 
 end :: Bool -> IO ()
-end b = putStr $ Stream.add [("success", b)] ++ Stream.end
+end b = putStr $ Stream.add (Stream.JSONObject [("success", b)]) ++ Stream.end
 
 note :: String -> IO ()
-note s = add [("note", s)]
+note s = add (Stream.JSONObject [("note", s)])
 
-add :: JSON a => [(String, a)] -> IO ()
-add v = putStr $ Stream.add v
+add :: Stream.JSONChunk a -> IO ()
+add = putStr . Stream.add
 
-complete :: JSON a => [(String, a)] -> IO ()
+complete :: Stream.JSONChunk a -> IO ()
 complete v = putStr $ Stream.start v ++ Stream.end
 
 -- A value that can be displayed either normally, or as JSON.