output/wavedrom: separate data processing logic from init/cleanup

Rearrange the order of routines in the wavedrom output module. Keep the
flow of .receive() -> .process_logic() -> .wavedrom_render() in one common
group of routines, which is not disrupted by the .init() and .cleanup()
routines which are kind of boilerplate in the source file. This increases
readability and maintainability.