Syllabus next steps: June edition

It's a relief to get through this particularly challenging corner of the syllabus:

I think the ones I'll look at next, in no particular order, are:
- [x] Dates & Times
- [x] Regular Expressions
- [x] Iterators
- [x] Dict functions (see comments in #812)
- [x] Higher-Order Functions

There is another difficult group, which I'm not ready to tackle yet:
- [x] N-Dimensional Arrays
- [ ] ~~Dataframes~~ _(not available in test runner!)_
- [x] Comprehensions (the full version)
- [x] Linear Algebra (in a tastefully restrained way, unless my impulses get the better of me!)

Longer-term, we need somewhere to fit in Macros (_aargh!_) and Modules.

Also, we still need an exercise to pair with the merged Rational Numbers concept.

These are some entries in #902, originally drafted by Erik long ago:

- [ ] `symbols`
- [x] `interfaces`
- [ ] `modules`
- [ ] `recursion`
- [ ] `tasks`
- [ ] `channels`
- [ ] `macros` (this should probably be split up into multiple concepts)

We've sort of talked a bit about symbols, I'm not sure how we'd expand this into a separate concept.

Recursion isn't especially popular in the Julia community, as far as I can tell.

I'd have to do a lot more reading before I can saying anything sensible about interfaces, tasks and channels. Getting into parallel programming, I guess: another big challenge.