Make interrupt module more standard.

- Move typelevel interrupts to a special-purpose mod: `embassy_xx::interrupt::typelevel`.
- Reexport the PAC interrupt enum in `embassy_xx::interrupt`.

This has a few advantages:
- The `embassy_xx::interrupt` module is now more "standard".
  - It works with `cortex-m` functions for manipulating interrupts, for example.
  - It works with RTIC.
- the interrupt enum allows holding value that can be "any interrupt at runtime", this can't be done with typelevel irqs.
- When "const-generics on enums" is stable, we can remove the typelevel interrupts without disruptive changes to `embassy_xx::interrupt`.

embassy-stm32/src/time_driver.rs

@@ -11,7 +11,7 @@ use embassy_time::driver::{AlarmHandle, Driver};
 use embassy_time::TICK_HZ;
 use stm32_metapac::timer::regs;
 
-use crate::interrupt::Interrupt;
+use crate::interrupt::typelevel::Interrupt;
 use crate::pac::timer::vals;
 use crate::rcc::sealed::RccPeripheral;
 use crate::timer::sealed::{Basic16bitInstance as BasicInstance, GeneralPurpose16bitInstance as Instance};