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/usb/mod.rs

@@ -1,4 +1,4 @@
-use crate::interrupt::Interrupt;
+use crate::interrupt;
 use crate::rcc::RccPeripheral;
 
 #[cfg(feature = "nightly")]
@@ -13,7 +13,7 @@ pub(crate) mod sealed {
 }
 
 pub trait Instance: sealed::Instance + RccPeripheral + 'static {
-    type Interrupt: Interrupt;
+    type Interrupt: interrupt::typelevel::Interrupt;
 }
 
 // Internal PHY pins
@@ -29,7 +29,7 @@ foreach_interrupt!(
         }
 
         impl Instance for crate::peripherals::$inst {
-            type Interrupt = crate::interrupt::$irq;
+            type Interrupt = crate::interrupt::typelevel::$irq;
         }
     };
 );