feat: add refcell1 exercise

[exdx]

Adds a basic exercise to introduce the concept of a RefCell. This API is not necessarily for beginners but is useful to have knowledge of.

[manyinsects]

I like the idea of teaching about RefCells, but I feel like this exercise would have to be somewhat more sophisticated... I'd like people to be able to see the difference of behaviour between the normal compile-time borrow check and what RefCell provides, maybe a more engaging exercise format could be converting something that's impossible with normal borrowing rules to using RefCell?

[exdx]

Thanks for taking a look @shadows-withal. I agree the example could be more sophisticated and I will look into updating it. Potentially following the Rust Book more closely is a good direction to take.

[exdx]

@shadows-withal I have updated the example to use the compile instead of test mode. I think this make the logic considerably simpler. The example is a straightforward example of using a RefCell to mutate via an immutable reference.

I think since this may be the first introduction to RefCell a user has, it's simplicity is a feature. Happy to adjust as necessary.

[mo8it]

The exercise teaches the usage of RefCell very well, but not really when to use it. It might be just confusing why the user shouldn't use &mut self since a RefCell is not really required in this example at all.

[exdx]

The exercise teaches the usage of RefCell very well, but not really when to use it. It might be just confusing why the user shouldn't use &mut self since a RefCell is not really required in this example at all.

I see -- that's fair. At the risk of making the example more convoluted I can update the example to provide a scenario where refcell is a necessity.

[mo8it]

I am a very new maintainer. Let's wait for the feedback from @shadows-withal first :D

[Piyuuussshhh]

I think exercises on Cell, RefCell, RwLock and Mutex in sequence would be nice. I remember there was a video that explained when these smart pointers become useful quite nicely. It explains why and when interior mutability is required.
The dude took the construction of a graph and updating the value of a node & its neighbors as the running example throughout the vid. If you want, I could link it!

PS: I'm sorry if exercises on these features have already been discussed and decided. I just finished all exercises and was wondering why these weren't included. BTW doing god's work, you guys! Thank you so much for creating this project!