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/build.rs

@@ -160,13 +160,11 @@ fn main() {
     }
 
     g.extend(quote! {
-        pub mod interrupt {
-            use crate::pac::Interrupt as InterruptEnum;
-            use embassy_cortex_m::interrupt::_export::declare;
+        embassy_cortex_m::interrupt_mod!(
             #(
-                declare!(#irqs);
+                #irqs,
             )*
-        }
+        );
     });
 
     // ========