Future concepts

This table is copied from #566. Things have moved on since Erik posted this in March 2023, but it still contains useful suggestions. I just added the checkmarks.

I probably wouldn't want all of these as separate concepts: it's too hard to create matching exercises. However, a lot of this could usefull be covered in some way.

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# Missing concepts

The following list of concepts is ordered by importance, starting with the most important concepts:

- [x] `loops`
- [x] `composite-types`
- [x] `mutating-functions`
- [x] `type-annotations`
- [x] `type-inference` (I think this goes well with `type-annotations`)
- [x] `char`
- [x] `bitwise-operations`
- [x] `vectorized-operators`
- [x] `conversion`
- [x] `promotion`
- [x] `constructors`
- [x] `mutable-composite-types`
- [x] `abstract-types`
- [x] `primitive-types`
- [x] `type-unions`
- [x] `sets`
- [ ] `symbols`
- [x] `exceptions`
- [ ] `interfaces`
- [ ] `modules`
- [ ] `recursion`
- [ ] `tasks`
- [ ] `channels`
- [ ] `macros` (this should probably be split up into multiple concepts)
- [x] `complex-numbers`
- [x] `rational-numbers`