dnd-character: add new exercise

[sshine]

This exercise was created in exercism/problem-specifications#1397.

It is a basic exercise in random generators, so the idea I had for the
Haskell track was to practice making QuickCheck generators rather than
simply use the standard random generator.

For this reason, QuickCheck is added as a root dependency.

[sshine]

@petertseng: How do I run the checks of Travis CI locally before submitting a PR?

Update: I see that you answered this in #751, e.g.:

bin/test-all-examples examples/dnd-character

---

My current workflow implies a lot of forth-and-back between the PR and Travis before succeeding.

It appears that it's the following lines from .travis.yml that triggers various failures:

for exercise in ${TRAVIS_BUILD_DIR}/exercises/*/ ; do
    time bin/test-stub $exercise
    time bin/test-all-examples $exercise
done

Perhaps this could be delegated to a script in bin/ that is compatible with both Travis and running it locally?

[petertseng]

I am hoping this exercise with randomness is easier to understand than the other two exercises that want it (simple-cipher and robot-name). I think it is because we know it's going to be Gen, whereas with the other two exercises we have IO and there are a lot of things that could be IO. (though hopefully students do know to search for random at some point. It's in the exercise descriptions after all)

If students still find it too hard, perhaps they would benefit from a link to Gen. And perhaps an exercise showing do-notation should be strategically placed, with links to explanations of that. If that is so, we'd want to record that last idea in #761 .

[sshine]

Thanks! I'll address your comments on Monday.

[sshine]

Since I haven't advanced this one yet, note to self:

• Make a .meta/hints.md for this exercise.

[sshine]

I've added some links in the hints section, changed the title, switched to RecordWildCards in the example and switched to printf in the test.