Review all practices/prerequisites arrays for practice exercises

[verdammelt]

Summary

This will be a WIP PR to work through #450. I assume that smaller PRs will be spawned off this (such as for new concepts).

My thought is to clear out all practices/prerequisites for exercises and review them individually. Either look at the exercise and determine the right answer for it; or look at configlet info to see what concepts are not listed anywhere and see if there is a place for them.

Must remember that a practice exercise does not require a non-empty practices or prerequisites array.

Possible future concepts

These are things noticed in practice exercises that likely warrant a concept:

• [ ] sequence (with functions like count) (could be practiced by hamming)
• [ ] binding (with forms such as let) (lots of exercises)
• [ ] iteration (perhaps more specific loop-for-as-arithmetic type of thing?) (several exercises)
• [ ] format-numbers (~D, ~X, ~R etc) (roman-numbers and say are perfect for this)
• [ ] signaling conditions (especially error signaling)