Syllabus: August update

This is an update to issue #956. Two months on, there has been some progress _(despite a subjective sense of wading through molasses!)_

- Both `dates-times` and `regular-expressions` are now live.
- Three new concepts are now live(ish), but still need exercises:
- [x] `multi-dimensional-arrays` : no idea of what exercise to use.
- [x] `comprehensions`: PR #992 is a draft which might fit with a bit more work.
- [x] `higher-order-functions`: no clear idea of an exercise, but might something inspired by #992 also be developed to fit here?

## At an earlier stage:
- [x] `linear-algebra` has a part-written concept that is getting _very long_. I need to finish that before worrying about an exercise.
- [x] `dict-functions` is at an early stage of doodling, to see how `python/mecha-munch-management` could be translated to Julia.

## Interfaces

There was some discussion about this in the context of `iterators`.

- For an iterators exercise, we have two to play with. I'm currently at an early stage of thinking about them.
- [ ] `python/plane-tickets`
- [ ] `julia/old-fibonacci iterator`

The [Julia docs](https://docs.julialang.org/en/v1/manual/interfaces/) list several other interfaces. Are any of these important enough for an Exercism syllabus?
- [ ] Indexing
- [ ] Abstract Arrays
- [ ] Strided Arrays
- [ ] Customizing broadcasting
- [ ] Instance Properties
- [ ] Rounding

## Documents

Reports of typos have become rare recently. I suspect we wore students down, rather than making everything typo-free!

References needs improving:
- Cross-links to other syllabus Concepts (now that most of them exist).
- Add more links to relevant bits of [Wikibooks](https://en.wikibooks.org/wiki/Introducing_Julia).

Anything else?