script_task, handle_msgs: sort for pattern constructor
script_trask, handle_msgs: factored out handle_msg_from_constellation
script_trask, handle_msgs: factored out handle_msg_from_script
script_trask, handle_msgs: factored out handle_msg_from_devtools
auto merge of #4200 : payload/servo/refactor-vertical-align-1, r=cgaebel

This vertical alignment greatly increases readability of this 22 arms match statement. To assess the readability I tried to read the first argument to the constructors in each pattern, e.g.  `FireTimerMsg` in `FromScript(FireTimerMsg(FromWorker, _))`, and concluded the readability was awful before and readable afterwards.