Auto merge of #16861 - gterzian:use_microtask_to_await_stable_state, r=jdm

Use microtasks to await a stable state.

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@jdm @KiChjang First pass at using microtasks to await a stable state. I ran into all sorts of problems to get it to compile, I think it's mainly related to the fact that the microtasks are stored in a `Vec`, which meant the `Runnalbe.handler(self: Box<Self>)` couldn't be called while iterating over the Vec... It compiles now although I haven't run any tests. I'm assuming I'm missing something fundamental and was hoping my changes would highlight the problems I run into, and you had a better idea of how to implement this... Perhaps we shouldn't pass a `Runnable` to `await_stable_state` at all?

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15375 (github issue number if applicable).

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