[WIP/PoC] Add async testbench functions

[zyp]

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 39.42308% with 63 lines in your changes are missing coverage. Please review.

Project coverage is 88.84%. Comparing base (7445760) to head (6faab6e).

Files	Patch %	Lines
amaranth/sim/core.py	43.37%	38 Missing and 9 partials :warning:
amaranth/sim/_pycoro.py	15.78%	15 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #990      +/-   ##
==========================================
- Coverage   89.39%   88.84%   -0.56%     
==========================================
  Files          43       43              
  Lines        9516     9611      +95     
  Branches     2283     2302      +19     
==========================================
+ Hits         8507     8539      +32     
- Misses        820      874      +54     
- Partials      189      198       +9     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[zyp]

I'm not intending to commit to this approach, but I think it'd be useful to prototype a simple working implementation first and write a suite of passing tests against it before starting on the «real» implementation.

[whitequark]

Yeah, no objection to that, of course.

[whitequark]

Thank you for this work, @zyp! PR #1344 supersedes this PR with a more complete implementation. We have ended up rewriting a lot of the simulator internals, as well as filling in the blanks in a few places where the RFC was unimplementable or underspecified.