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).
The UARTETWISPIn naming is quite horrible. With the nRF53, Nordic realized this
and renamed the interrupts to SERIALn. Let's copy that for our peripheral names, in nrf53 and nrf91.
