Buttons

Takes user input and updates the input registers

Standard keyboard controls:

D-Pad -- arrow keys
B -- Z
A -- X
Start -- Enter
Select -- Space

`init()`

Set up the intial state of each button (nothing pressed)

`tick()`

if any button is pressed which wasn't pressed last time, interrupt
every tick: update_buttons()
once per frame: handle_inputs()
TODO: do we also need to interrupt on button release?
TODO: do we also need to interrupt even when neither Dpad nor Buttons are selected?

`update_buttons()`

update the I/O registers based on which buttons are currently held

Since the hardware uses 0 for pressed and 1 for released, we invert on read and write to keep our logic sensible....

`handle_inputs()`

Accept input events from SDL