Need API to get an available PIO state machine

[niklauslee]

It is required when using multiple PIO libraries at the same time.

const {StateMachine} = require('rp2');
// ...
var sm = new StateMachine(StateMachine.getAvailableId(), ...);
// The sequence of StateMachine'id is [0, 4, 1, 5, 2, 6, 3, 7]