1321: executor: add Pender, rework Cargo features. r=Dirbaio a=Dirbaio
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).
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>
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