WIP: add prerequisites to practice exercises

[cmcaine]

Closes #314.

I've done the first few and welcome feedback on whether they're okay.

As I'm sure you know, we don't have enough concepts in config.json to do this at the moment.

Even including all the ones in the concepts dir, we're still missing a concepts for:

• loops
• exceptions
• common data structures (dicts, vectors, sets, tuples) (I wonder if there should be a parent concept of "common data structures" to avoid revealing which ones to use?)
• structs
• bitwise operators

I've just picked likely names and put them in prerequisites without adding them elsewhere.

There's also the issue of multiple solutions to problems. Should I add a concept as a prereq if it is commonly used in correct solutions, or only if it is required?

And I'm assuming I should avoid adding concepts that are parents of concepts that are required.

[cmcaine]

I've resolved the merge conflicts.

I don't know how all of this interacts with us not having concept exercises turned on. Maybe we should move all of these prerequisites to practices or something instead?