Make `bits` pub?

[astro]

I've got 4 LEDs that I can manipulate individually. My first try looks like this:

fn toggle_led(led: u8) {
    match led {
        1 => 
            gpio_port().not[2].write(|notw| notw.notp1(true)),
        2 => 
            gpio_port().not[2].write(|notw| notw.notp2(true)),
        3 => 
            gpio_port().not[2].write(|notw| notw.notp8(true)),
        4 => 
            gpio_port().not[5].write(|notw| notw.notp26(true)),
        _ => (),
    }
}

It's hard to parameterize over notp1, notp2, etc. In this case I'd like to sacrifice the pretty abstraction and manipulate any bit of self.bits.

[japaric]

We can add an unsafe accessor to get &mut bits.

[japaric]

We can can't make bits public because that would make it "safe" to modify bits in a way that breaks the invariants that the abstraction preserves.