Fix mangling of --json output of utf-8 characters when not running in a utf-8 locale

As long as all code imports Utility.Aeson rather than Data.Aeson,
and no Strings that may contain utf-8 characters are used for eg, object
keys via T.pack, this is guaranteed to fix the problem everywhere that
git-annex generates json.

It's kind of annoying to need to wrap ToJSON with a ToJSON', especially
since every data type that has a ToJSON instance has to be ported over.
However, that only took 50 lines of code, which is worth it to ensure full
coverage. I initially tried an alternative approach of a newtype FileEncoded,
which had to be used everywhere a String was fed into aeson, and chasing
down all the sites would have been far too hard. Did consider creating an
intentionally overlapping instance ToJSON String, and letting ghc fail
to build anything that passed in a String, but am not sure that wouldn't
pollute some library that git-annex depends on that happens to use ToJSON
String internally.

This commit was supported by the NSF-funded DataLad project.

Key.hs

@@ -22,7 +22,6 @@ module Key (
 	prop_isomorphic_key_decode
 ) where
 
-import Data.Aeson
 import Data.Char
 import qualified Data.Text as T
 
@@ -30,6 +29,7 @@ import Common
 import Types.Key
 import Utility.QuickCheck
 import Utility.Bloom
+import Utility.Aeson
 import qualified Utility.SimpleProtocol as Proto
 
 stubKey :: Key
@@ -155,8 +155,8 @@ instance Hashable Key where
 	hashIO32 = hashIO32 . key2file
 	hashIO64 = hashIO64 . key2file
 
-instance ToJSON Key where
-	toJSON = toJSON . key2file
+instance ToJSON' Key where
+	toJSON' = toJSON' . key2file
 
 instance FromJSON Key where
 	parseJSON (String t) = maybe mempty pure $ file2key $ T.unpack t