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_otg/mod.rs

@@ -1,7 +1,5 @@
-use embassy_cortex_m::interrupt::Interrupt;
-
-use crate::peripherals;
 use crate::rcc::RccPeripheral;
+use crate::{interrupt, peripherals};
 
 #[cfg(feature = "nightly")]
 mod usb;
@@ -25,7 +23,7 @@ pub(crate) mod sealed {
 }
 
 pub trait Instance: sealed::Instance + RccPeripheral {
-    type Interrupt: Interrupt;
+    type Interrupt: interrupt::typelevel::Interrupt;
 }
 
 // Internal PHY pins
@@ -109,7 +107,7 @@ foreach_interrupt!(
         }
 
         impl Instance for peripherals::USB_OTG_FS {
-            type Interrupt = crate::interrupt::$irq;
+            type Interrupt = crate::interrupt::typelevel::$irq;
         }
     };
 
@@ -161,7 +159,7 @@ foreach_interrupt!(
         }
 
         impl Instance for peripherals::USB_OTG_HS {
-            type Interrupt = crate::interrupt::$irq;
+            type Interrupt = crate::interrupt::typelevel::$irq;
         }
     };
 );