Why:
* We use immutable.js 3.8.2 which is ancient and not getting updates
* immutable.js adds 55KB to the bundle, of which we use almost nothing
* immutable.js's treatment of 0 and -0 is nonsensical and would require weird workarounds like https://github.com/TurboWarp/scratch-vm/pull/320
* We don't benefit from the whole immutability thing
What we're doing instead:
* We have our own MonitorState and MonitorRecord classes
* We track dirty-ness at modification time instead of equality checking every tick
* scratch-gui and packager can continue to think we use immutable.js without causing issues in either direction
* our objects have enough stubs that immutable.js-expecting consumers are ok
* when receiving an object, we check if it looks like an immutable.js one and ask for the plain JS version so that works okay too
Thomas Weber