Add resistor-color exercise

[still-flow]

Ported over from the C track, but with a Rust-ic flavor. Hopefully the external crate usage is okay.

[coriolinus]

Just took a quick look; it's definitely a good start! Some notes:

• You're going to need to edit the top-level config.json to add the exercise there as well.
• What is this exercise meant to teach?
• If we can come up with a good educational goal, then could we structure this exercise as a concept exercise instead of a practice?

[still-flow]

Re what this exercise teaches: I think it teaches about C-like enums, also about external crate usage. And the neat "{:?}" formatting trick.

As for making this a concept exercise: sure! It's just that I'm new to contributing exercises, so a practice one is certainly easier. However, I'll probably be able to figure things out if necessary.

[still-flow]

Re editing top-level config.json: where should I read to learn how to properly do it?

[still-flow]

Well, here's my attempt, anyway. Please feel free to correct me.

[coriolinus]

Some more notes below.

The biggest comment I have is that we have to decide now if this is going to be a concept exercise; it's not something that can be changed after the exercise goes live. This seems plausible to me for teaching the external_crates concept. Upside: it gets a lot more visibility, as the concepts in Rust still need a lot of fleshing out. Downside: creating the concept requires a fair amount of extra work. If you do decide to convert it to a concept exercise, it should use the enum-iterator crate.

Concept docs, concept exercise docs.

If you don't want to do the extra work, this is also fine as a practice exercise. Just let me know whether you want to convert this or not.

[still-flow]

Okay, here's all the fixups for now. I think I should try converting this to a concept exercise, but failing that, this should be good for now.

[coriolinus]

Cool, I'll look forward to seeing how this looks as a concept!

[still-flow]

Alright, here is a rough first version of that. (To be clear, some of the text was liberally adapted from Rust's docs.)

[coriolinus]

This looks very plausible as a concept to me!

[still-flow]

Alright, re-adding the int-enum crate seems to have worked.

[coriolinus]

Looks good! Thanks for putting in the effort to push this over the finish line.

[still-flow]

It works! Albeit not without trouble: it would seem that the crates have to be re-fetched on every test instance, which slows tests down considerably, even leading to timeouts sometimes. That's probably why there was a rule against external crates?

[coriolinus]

Hmm, I'm pretty confident that it's not re-fetching the crates each time, because the test runners run offline. That's why there's a list of supported crates in the first place. However, it's worth looking into the timeout rate.