This fixes #45.
For every Method/Notification enum we can either specify an arbitrary expression to evaluate when handling a message or use a default handler following existing convention:
this.handler.<action_name>([id,] params, &*this.output).
Few things to note:

• I'd love to get rid of trace_params as I think there may be better idea to handle an optional enum payload token in trace! call
• Possibly lines with trace! call are too long, but I figured it doesn't add to the logic itself, so throwing a one-liner just to inform of a trace is acceptable
• I defined macros inside impl for LsService, as the macro is closely related only to that particular scope and it seemed somewhat self-contained, but if needed I can move it to top-level so it generates whole LsService::handle_message (along with initial parse_message, shut_down logic etc.)