feat: add exercise involving lifetime elision

[sgasse]

No description provided.

[sgasse]

Hey @shadows-withal ! Your talk yesterday at EuroRust inspired me to contribute this example 🙂 A colleague of mine asked for help with a much more cluttered real-life example of the same problem that he ran into. Hope it can be useful to others as well.

[sgasse]

I integrated your suggestions and updated the wording to be more concrete. Could you take another look @shadows-withal ?

[mo8it]

I really like this exercise. It needs to be adapted to version 6 by adding a solution, adapting the info in info.toml, adding TODOs and removing the hint about getting a hint and I AM DONE.

[sgasse]

OK nice, I will rebase and push an updated version 👍

[sgasse]

Thank you @mo8it for reading through this change - I recreated the example on top of latest main and adjusted to version 6 following your instructions. Could you check again please?

Also, I may be missing something obvious here, but I was not able to test following the suggestions from CONTRIBUTING.md. There it says:

Make sure your exercise runs with rustlings run yourTopicN

However, I get the following error in this case:

> rustlings run lifetimes4
Error: You are trying to run Rustlings using the old method before version 6.
The new method doesn't include cloning the Rustlings' repository.
Please follow the instructions in `README.md`:
https://github.com/rust-lang/rustlings#getting-started

Though cargo run -- run lifetimes4 and cargo run -- hint lifetimes4 work as expected so I hope the patch follows version 6 correctly. What is the best way to run rustlings with an updated state of the exercises? 🤔