refactor(s2n-quic-core): simplify IO traits

[camshaft]

Description of changes:

While implementing the AF_XDP IO provider, I wanted to make a generic event loop implementation so I didn't have to copy/paste the one from the tokio provider.

In this change, the core IO traits are refactored to make it possible to use in the generic IO provider event loop.

Call-outs:

The previous versions of the IO traits assumed that all of the incoming and outgoing packets were addressable through a slice. This was done to integrate with something like rayon to perform packet crypto in parallel. The problem is that assumption isn't really the case with AF_XDP, where we're putting descriptors in a ring buffer and they're not dereferenceable without the UMEM. I also think there's other ways to perform packet encryption in parallel that don't force the IO provider to adhere to this strict interface, so it's best to keep it simple and put the complexity elsewhere (especially since it's not even being used right now.)

I've also opened #1742 to highlight a better interface using GATs, which are only available in rust >=1.65. We can change the API once the MSRV bumps.

Testing:

Since this is just shuffling things around and simplifying traits, the existing tests should cover the changes.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[camshaft]

Running into an integration issue with needing generic associated lifetimes... Going to see if I can rework it to avoid it.

[camshaft]

This gist outlines the issue and the potential work-around. Going to investigate if this is safe or not.

[camshaft]

I think we're OK to use the method as outlined above. I will include a comment and open an issue.