Remove immutable.js

[GarboMuffin]

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 Fix negative zero in monitors #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