Since https://github.com/rust-lang/rust/issues/96992 has stalled,
to prevent potential unsoundness caused by transmuting to &WakerHack,
we can use nightly waker_getters APIs by gating it behind nightly
feature in embassy-executor without waiting for it to be stablized.
* Feature auto-documentation
* Task arena sizes in a <details> list
* Non-documented comment explaining turbowakers with see-also link
Further improvements:
* Are the task-arena-size-* numbers sizes in bytes? or something else?
* Task arena section could benefit from advice about how to choose a
suitable size
This introduces a `Pender` struct with enum cases for thread-mode, interrupt-mode and
custom callback executors. This avoids calls through function pointers when using only
the thread or interrupt executors. Faster, and friendlier to `cargo-call-stack`.
`embassy-executor` now has `arch-xxx` Cargo features to select the arch and to enable
the builtin executors (thread and interrupt).
