smol: Improve implementation and make stable

In #1901 we discussed that async-std is discontinued and we wanted an implementation for smol. The MR for that implementation will soon be merged: !2986 (merged)

We follow-up the development of the smol-implementation in this issue, with the goal to improve it and make it stable so it can serve as a worthy alternative to tokio.

Feel free to add more TODOs required to improve the current smol implementation.

TODO:

• Make smol multi-threaded.
• Add functions to create a smol runtime from an existing smol executor.
• Add more documentation and examples demonstrating how to use smol in Arti.
• (@opara added) Investigate if we can reduce the size of the internal CompoundRuntime (see !2986 (comment 3243972)).