I've had to change the output of `git annex metadata --json`.
The old output looked like this:

	{"command":"metadata","file":"foo","key":"...","author":["bar"],...,"note":"...","success":true}

That was not good, because it didn't separate the metadata fields
from the rest of the JSON object. What if a metadata field is named
"note" or "success"? It would collide with the other "note" and "success"
in the JSON.

So, changed this to a new format, which moves the metadata fields into
a "fields" object:

	{"command":"metadata","file":"foo","key":"...","fields":{"author":["bar"],...},"note":"...","success":true}

I don't like breaking backwards compatability of JSON output, but in this
case I could see no real alternative. I don't know if anyone
is using `metadata --batch` anyway. If you are and this will cause a
problem, get in touch.

----

While making that change, I also improved the JSON outputlayer, so it can
use Aeson. I didn't switch exclusively to using Aeson, because git-annex
commands with JSON output often output it incrementally as they go (which
Aeson can't really do), and are anyway not often doing the kind of
serialization of data type that Aeson excells at.

This let me use Aeson to generate the "fields" object for `metadata
--json`. And it was also easy enough to use Aeson to parse the output of
that command (and some simplified forms of it).

So, I've laid the groundwork for `git annex metadata --batch` today.