Add reverse-string exercise

[BNAndras]

Closes #641

[ceddlyburge]

Looks good to me! I'll leave others a bit of time in case they want to review, but if not I'll merge it next weekend (which I think is a week before it is needed for the 48 in 24 challenge)

[jiegillet]

Thanks for the contribution, looks good.

Just a note, I'm not a fan of this exercise, since it's too basic (standard lib function). In the Elixir track, we decided not to implement it for the same reason. I don't mind merging too much though, since the 48in24 people will expect it 🤷

[ceddlyburge]

I do agree that this is a pointless exercise, and it seems like a strange choice for the 48 in 24. Maybe some languages don't have an equivalent of String.reverse, in which case we could ask students to solve it without using the library function?

[BNAndras]

I could add an instructions append to recommend not using the standard library. Strain has similar text in the instructions that we can adapt. I avoided using the built-in reverse to show the exercise is fairly straightforward even without it.

[ceddlyburge]

Adding something to the instructions sounds like a good idea to me.

We could even enforce it with the analyzer. Not sure it is worth it though.

[BNAndras]

I agree. I'm skeptical that anything to curtail standard library use beyond the append would justify the effort spent. If an append won't suffice, I'm fine not porting the exercise.

[ceddlyburge]

Adding something to the instructions works for me ...

[jiegillet]

I don't think this requires adding special instructions, it's not worth it. List.reverse is how you'd solve that problem, so be it. Quick and easy exercise.
If you want to implement interesting list-style operations, we have list-ops.