Replace nostd_async with a simple test executor

[jannic]

This has some advantages:

• no longer depend on nostd_async, which has known issues (Soundness issues sammhicks/nostd_async#4) and looks rather unmaintained
• the custom executor can check that the waker is only called when expected (currently only implemented as some defmt logging, but this can be improved)
• in case of problems, it is easier to debug than a full-fledged executor

The new dependency on once_cell can be removed with rust 1.82.0, which makes Waker::from_raw callable from const contexts.

[jannic]

@ithinuel Thanks for the approval, it's encouraging that you agree with the approach. I may push some code cleanups to this pull request later today.

[sammhicks]

Hi, I'm the developer of nostd_async.

There was definitely UB in nostd_async, which should now have been resolved.

Miri complains about version 0.6.1, but not 0.7.0 to 0.7.2.

However, I'm no longer developing it, so using a different executor is wise.