This PR tries to fix #36 and part of #18 by not evaluating property getters during compilation.

Open tasks:

• Why is this example not working?

  let wait do
      await (core.debug == true)
      core.print('done')
  end

  When calling wait() and setting core.debug = true, there should be a message "done".
• Fix this example:

  serial = Serial(26, 27, 115200, 1)
  p0 = Expander(serial, 25, 14)
  b = p0.Input(5)
  b.pullup()
  
  when b.level == 0 then core.print("DOWN"); end
  when b.change > 0 then core.print("PRESS"); end

  This startup script fails, because the compiler needs to read b.level and b.change to get their datatype. But they are undefined until the expander has started.