Adds IRQ State types

[jonathanpallant]

Some bike shedding required on the names, but...

These types help you move a singleton from the main thread into an interrupt handler. The global variable is a Mutex<RefCell<Option>> and the local variable is designed to go into an #[interrupt] and is a plain Option that relies on the interrupt macro to give you an &mut Option<T>.

This application for the nRF52840 makes use of these types: https://github.com/ferrous-systems/rust-exercises/tree/7e6d9e6029ad9a31b0c75d233dda6921cd8421f0/nrf52-code/puzzle-fw

[thejpster]

@jamesmunns any thoughts on this one?

[jamesmunns]

@thejpster I haven't had the chance to fully digest this, since this isn't a data structure or pattern I typically use in any of my projects.

It seems sort of like a oneshot channel plus a lazy static, and a lazy static that specifically requires the cortex-m interrupt transform that is only sound on single core devices?

[thejpster]

That's a fair first approximation. The caching into an IRQ-local variable saves you the cost of a lock to peek in the channel on subsequent interrupts.

I have thoughts on static mut I'll put in the ticket.