executor: add Pender, rework Cargo features.

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).

examples/stm32f0/src/bin/multiprio.rs

@@ -62,7 +62,7 @@ use core::mem;
 use cortex_m::peripheral::NVIC;
 use cortex_m_rt::entry;
 use defmt::*;
-use embassy_stm32::executor::{Executor, InterruptExecutor};
+use embassy_executor::{Executor, InterruptExecutor};
 use embassy_stm32::interrupt;
 use embassy_stm32::pac::Interrupt;
 use embassy_time::{Duration, Instant, Timer};