Allow build executor

[ararog]

It would be nice to allow build a executor with minimal settings like tokio do, at least worker_threads and max_blocking_threads.

I been trying to run pyo3 with smol and I noticed a substantial difference when I lower down both on tokio. It would nice to easily do the same on smol.

[nanoqsh]

You can use this helper crate https://docs.rs/smol-macros/latest/smol_macros/

Yes, it looks like there is no option to specify the number of threads here. It might be useful to add this.

Personally, I don't like this solution with macros (in tokio too), I prefer to explicitly spawn all threads

[ararog]

I believe the best option to pick here is either by contributing with a RuntimeBuilder to smol or create a separate crate to provide it.