switch MetaValue to ByteString and MetaField to Text

MetaField was already limited to alphanumerics, so it makes sense to use
Text for it.

Note that technically a UUID can contain invalid UTF-8, and so
remoteMetaDataPrefix's use of T.pack . fromUUID could replace non-UTF8
values with '?' or whatever. In practice, a UUID is usually also text,
I only kept open the possibility of it containing invalid UTF-8 to avoid
breaking parsing of strange UUIDs in git-annex branch files. So, I
decided to let this edge case slip by.

Have not updated the rest of the code base yet for this change, as the
change took 2.5 hours longer than I expected to get working properly.

Utility/Aeson.hs

@@ -44,6 +44,9 @@ encode = Data.Aeson.encode . toJSON'
 class ToJSON' a where
 	toJSON' :: a -> Value
 
+instance ToJSON' T.Text where
+	toJSON' = toJSON
+
 instance ToJSON' String where
 	toJSON' = toJSON . packString