replace tweetNaCL exercise with Qoi interface

[squell]

This is a backport from rust-training material.

[hdoordt]

Great work on the exercise! Looks really good.

I don't see why this needs to replace the TweetNaCl exercise, though. It's nice for people to be able to pick more exercises

[hdoordt]

Please use the #[modmod:exercise_ref] to enable modmod to number the exercises upon generating. Example

[squell]

Would it be possible to hack modmod so it just 'sees' that or any markdown subheading? Or is there use case why we would want to have unnumbered subheadings?

[squell]

Great work on the exercise! Looks really good.

I don't see why this needs to replace the TweetNaCl exercise, though. It's nice for people to be able to pick more exercises

I mean sure if you want, we can keep it, but the tweetnacl exercise has some didactic problems:

• There's a extern declaration for a randombytes() in tweetnacl.c that's not provided. On Linux/MacOS that turns out to not be an issue if you don't reference the functions that call it, but on Windows we've seen people run into problems.

• tweetNaCL is actually a very poor example of a C library. If people think, well that's nice, let's also implement crypto_hash_sha256_tweet you're off into the deep end (you notice there's not a literal definition of crypto_hash_shaXYZ_tweet. If you persevere you see that you can make it work by editing tweetnacl.h in exactly the right way---but then you will loose the implementation for crypto_hash_sha512_tweet. In tweetNaCL, a lot of macro trickery was involved to squeeze it in 100 tweets and this is the downside.

• There's not really a tangible way for students to feel that the bindings are actually doing something useful since by definition a SHA512 will just be a collection of 64 meaningless bytes. With Qoi it's pretty rewarding: you get an image.