Update interrupt macro to match cortex-m-rt

[peacememories]

I think it is worth it to have a similar API surface to cortex-m-rt, since it seems to be the most popular embedded abstraction in rust at the moment. The crate uses procmacros to define attributes (like #[interrupt]) instead of macro_rules!, which might result in some slightly cleaner code.

Of course this would mean creating an additional crate, since afaik procmacro crates cannot expose anything else.

[Rahix]

Closing this in favour of Rahix/avr-device#1