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/dma/gpdma.rs

@@ -56,8 +56,8 @@ static STATE: State = State::new();
 pub(crate) unsafe fn init(irq_priority: Priority) {
     foreach_interrupt! {
         ($peri:ident, gpdma, $block:ident, $signal_name:ident, $irq:ident) => {
-            crate::interrupt::$irq::set_priority(irq_priority);
-            crate::interrupt::$irq::enable();
+            crate::interrupt::typelevel::$irq::set_priority(irq_priority);
+            crate::interrupt::typelevel::$irq::enable();
         };
     }
     crate::_generated::init_gpdma();