Add embassy-cortex-m crate.

- Move Interrupt and InterruptExecutor from `embassy` to `embassy-cortex-m`.
- Move Unborrow from `embassy` to `embassy-hal-common` (nothing in `embassy` requires it anymore)
- Move PeripheralMutex from `embassy-hal-common` to `embassy-cortex-m`.

embassy-stm32/build.rs

@@ -96,8 +96,9 @@ fn main() {
     g.extend(quote! {
         pub mod interrupt {
             use crate::pac::Interrupt as InterruptEnum;
+            use embassy_macros::cortex_m_interrupt_declare as declare;
             #(
-                embassy::interrupt::declare!(#irqs);
+                declare!(#irqs);
             )*
         }
     });