PIO timing improvements and dual MCU demo

[c1570]

This makes the PIOs run in sync with the ARM cores and enables ClockDiv.
This also adds a new demo program that runs two rp2040js instances with the two RP2040s wired together using a bus.

[c1570]

@urish From my side, this is ready for merging.

[urish]

Update: I'm working on a new simulation clock implementation that makes it easier to keep the peripherals synchronized accurately, and will also be more GC friendly. Plan to publish it shortly under "accurate-clock" branch - stay tuned

[urish]

Note: pushed the new clock implementation to the accurate-clock branch - see #137