exercism/crystal

* Updates to generators

* Update generators to use JSON.mapping

The new site lets people filter optional exercises by topic, and this
makes it easy for people to skip past them.

This is important since the math-y exercises are not key to learning
the language, and a lot of people find them intimidating and demotivating.

This updates any 'mathematics' topics to be 'math' since it's shorter
and won't get cropped in the UI.

It also removes the math topics for any exercises that are not in the
canonical math-y list.

This is a tiny formatting PR to move the sentences to separate lines.  Part of #117 and exercism/meta#85.

This adds the auto_approve property to the hello-world
exercise as part of preparing tracks for the v2 launch.

The current code on the server will automatically approve
hello-world exercises anyway, but that is a temporary fix
and will be removed in the future.

Reformat config files

This runs the configlet fmt command, which normalizes the
contents and ordering of keys in the track config and
maintainers config.

This will let us script changes to the config files without
having unnecessary noise in the diffs when submitting pull
requests.

…nerator

Fix difference_of_squares generator

…rator

Fix run_length_encoding generator

* trigger prerequisites from related make targets

While attempting to figure out how to add an exercise, I noticed that
some of the build targets defined in the Makefile were not automatic
(e.g. the generator build -> `generator/bin/generator`), so I added a
few prerequisites and their corresponding targets, along with a helper
target or two.

None of this is strictly necessary, of course. Please feel free to
close this PR with impunity if you see fit.

* remove brace expansion as it may break in POSIX shells

* include details re: makefile prereqs and clean target in the readme

* give configlet the explicit track-id and silence recursive make call

In the majority of exercism crystal exercises all but the first test are marked as pending.
That was not the case for the exercise difference_of_squares.
This commit amends that exercise so it matches the rest.

* bob: Lock description to match current implementation

The upstream description of 'bob' adds a new rule, which is not
yet reflected in the test suite in this track.

This locks the description of this exercise to match the current
test suite so that we can regenerate the exercise README without
having to worry about getting out of sync.

* Regenerate exercise READMEs to get most recent descriptions

The changes are purely stylistic: formatting, whitespace, and a couple of small copy tweaks that
clarify the exercise.

This gives us a clean diff in the next step.

* Inline the exercise README insert

Now that we have a track-wide template, we don't need a separate insert.

triangle: Generate missing exercise README