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/multicore.rs

@@ -50,7 +50,7 @@
 use core::mem::ManuallyDrop;
 use core::sync::atomic::{compiler_fence, AtomicBool, Ordering};
 
-use crate::interrupt::Interrupt;
+use crate::interrupt::InterruptExt;
 use crate::peripherals::CORE1;
 use crate::{gpio, interrupt, pac};
 
@@ -156,7 +156,7 @@ where
 
         IS_CORE1_INIT.store(true, Ordering::Release);
         // Enable fifo interrupt on CORE1 for `pause` functionality.
-        unsafe { interrupt::SIO_IRQ_PROC1::enable() };
+        unsafe { interrupt::SIO_IRQ_PROC1.enable() };
 
         entry()
     }