Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

Side exercises can only be unlocked by core exercises. Previously we
had exercises being unlocked by both core and side exercises.

Each core exercise should unlock at least one exercise. Previously we
had some core exercises that didn't unlock any side exercises. Now
that's only true of the last core exercise in the track.

Update config.json to set which exercises are core and which ones are
unlocked by other exercises.

The slug is not used anywhere. We initialize a track based on knowing the Track ID.
Since the repository is always named after the track ID, this field, too, is redundant,
as it can be inferred.

We will be moving to a tree-shaped track rather than a linear one, as described in the [progression & learning in Exercism](https://github.com/exercism/docs/blob/master/about/conception/progression.md) design document.

In order to support this, we need to expand the metadata that exercises are configured with.

Note that 'core' exercises are never unlocked by any other exercises. Core exercises appear in the track in the order that they are listed in the array.

Non-core exercises depend on only one exercise (unlocked_by: ). At the moment we are assuming that this is a core exercise, but there is no reason that it needs to be.

Until now we have operated with a separate deprecated array. These are now being moved into the exercises array with a deprecated field.

With these defaults the track in nextercism will have no core exercises, and all the exercises will be available as 'extras' from the start.

If you haven't already, now would be a good time to do the following:

* add a rough estimate of difficulty to each exercise (scale: 1-10)
* add topics to each exercise
* choose *at most 20 exercises* to be core exercises (set core: true, and delete the unlocked_by key)
* for each exercise that is not core, decide which exercise is the prerequisite (max 1)

If possible, leave 3 or 4 simple exercises as (core: false, unlocked_by: null), as this will provide new participants with some exercises that they can tackle even if they have not finished the first core exercise.

Removed the comma and set the difficulty to 1. :)

Put the slug at the end of the file

Updated the difficulty of the slug

Based on v1.0.0 of the canonical data:

https://github.com/exercism/problem-specifications/tree/28a32203b96ebaecac6a6968831bd66718d8ba30/exercises/list-ops

Plus modifications that are being discussed in

https://github.com/exercism/problem-specifications/issues/826

The leading 'x' is kind of arbitrary. Especially now that we can set
topics on the repositories, we don't need a pattern to distinguish what
is a track or not.

The repository itself has already been renamed. GitHub redirects from
the old name to the new name, so we do not have to rush to fix links to
the old repository name, though we should update them for the sake of
clarity.

Since the exercise implementations are all in the exercises directory
we no longer need to ignore any non-exercise directories in the root
of the track.

 This will let us script changes to it with less noise in the diffs.