Fill all exercises with accurate "practices" and "prerequisites" concepts

[jiegillet]

Now that we have more concepts, we should tag exercises with them (like in #629).

The "practices" should only have one, maybe two, core concept that will help the students master a specific concept.

The "prerequisites" are the concepts necessary to finish the task. This is not an exact science, a rule of thumb is looking at the concepts used in the examples solution.

Using bin/configlet info provides valuable information on concepts that are not practiced or prerequisites.
Ideally, all concepts should be practiced, although it's not always possible (concepts like phantom-types don't really make sense for one-module exercises).
We should definitely not have any "Concepts that are a prerequisite, but aren't practiced by any Practice Exercise".

Note: this does not need to be done in one go, incremental improvements, per exercise or per concept are very welcome.