Protect forbidden reopenings of classes/modules

[jorgemanrubia]

#26 and #27 introduced several mechanisms based on detecting tampering attempts statically before commands were executed. This PR adds a new check against reopening forbidden classes. If a forbidden reopening materializes, it will log the command and force exiting from IRB.

Take this example:

def my_constant
  ActiveRecord
end

class my_constant::Base
  def fake_save!(*args)
    puts "ActiveRecord::Base#save! overridden!"
  end
end

This code would skip our static analysis. We can detect it now because we let the code run and we validate that no new methods were added to the class. Because the code executes, the damage is done, so we log the attempt (which could be prevented by the command) and, in any case, we exit from IRB.