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-rp/src/uart/buffered.rs

@@ -3,14 +3,14 @@ use core::slice;
 use core::task::Poll;
 
 use atomic_polyfill::{AtomicU8, Ordering};
-use embassy_cortex_m::interrupt::{self, Binding, Interrupt};
 use embassy_hal_common::atomic_ring_buffer::RingBuffer;
 use embassy_sync::waitqueue::AtomicWaker;
 use embassy_time::{Duration, Timer};
 
 use super::*;
 use crate::clocks::clk_peri_freq;
-use crate::RegExt;
+use crate::interrupt::typelevel::{Binding, Interrupt};
+use crate::{interrupt, RegExt};
 
 pub struct State {
     tx_waker: AtomicWaker,
@@ -485,7 +485,7 @@ pub struct BufferedInterruptHandler<T: Instance> {
     _uart: PhantomData<T>,
 }
 
-impl<T: Instance> interrupt::Handler<T::Interrupt> for BufferedInterruptHandler<T> {
+impl<T: Instance> interrupt::typelevel::Handler<T::Interrupt> for BufferedInterruptHandler<T> {
     unsafe fn on_interrupt() {
         let r = T::regs();
         if r.uartdmacr().read().rxdmae() {