[WIP] Sketch out an API for managing interrupts

[Rahix]

Example usage:

let pca_irq = todo!("GPIO pin of the MCU");
let mut pca9555 = port_expander::Pca9555::new(i2c, true, false, false);
let pca_pins = pca9555.split();

loop {
    // Wait for an interrupt
    while !pca_irq.is_low() {}

    // Read info
    let changed: [bool; 2] = port_expander::fetch_pin_change([&pca_pins.io0_0, &pca_pins.io0_3]).unwrap();
}