extras: Fix UB in Peripheral

`Peripheral` assumed that interrupts can't be preempted,
when they can be preempted by higher priority interrupts.
So I put the interrupt handler inside a critical section,
and also added checks for whether the state had been dropped
before the critical section was entered.

I also added a `'static` bound to `PeripheralState`,
since `Pin` only guarantees that the memory it directly references
will not be invalidated.
It doesn't guarantee that memory its pointee references also won't be invalidated.

There were already some implementations of `PeripheralState`
that weren't `'static`, though,
so I added an unsafe `PeripheralStateUnchecked` trait
and forwarded the `unsafe` to the constructors of the implementors.

embassy-extras/Cargo.toml

@@ -17,4 +17,5 @@ embassy = { version = "0.1.0", path = "../embassy" }
 defmt = { version = "0.2.0", optional = true }
 log = { version = "0.4.11", optional = true }
 cortex-m = "0.7.1"
+critical-section = "0.2.1"
 usb-device = "0.2.7"