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`.

examples/stm32f7/build.rs

@@ -1,9 +1,8 @@
 //! adapted from https://github.com/stm32-rs/stm32f7xx-hal/blob/master/build.rs
-use std::env;
 use std::fs::File;
 use std::io::prelude::*;
-use std::io::{self};
 use std::path::PathBuf;
+use std::{env, io};
 
 #[derive(Debug)]
 enum Error {