Implement interrupts

This commit adds a new feature-flag `rt` which, when enabled, adds the
`#[interrupt]` procedural macro to define an interrupt handler.  Unlike
the implementation in cortex-m, this version needs an attribute which is
the name of the chip the interrupt is for.  In code, an interrupt
handler might look like this:

    #![feature(abi_avr_interrupt)]

    #[avr_device::interrupt(atmega32u4)]
    fn INT6() {
        // Do Something
    }

Closes #1.

Signed-off-by: Rahix <rahix@rahix.de>