[WIP] Sketch out alternative interrupts API

[Rahix]

An alternative to #28. This variant decouples the pins better.

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();

let p1 = pca_pins.io0_0;
let p2 = pca_pins.io0_3;

// Enable interrupts for these two pins
p1.listen().unwrap();
p2.listen().unwrap();

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

    // Fetch interrupt status (can be called on any pin of the expander)
    p1.fetch_all_interrupt_state().unwrap();

    let p1_changed: bool = p1.query_pin_change();
    let p2_changed: bool = p2.query_pin_change();

    // Calling the function again always yields false.  Only after a new
    // interrupt and fetch_all_interrupt_state() will you receive results again.
    assert_eq!(p1.query_pin_change(), false);
}