Improve waveform dumping, fix ast caching

• The vcd generation pass had some bugs and cannot handle some cases correctly. I took a pass to fix the bugs and refactored the code.
• Merge in Kaishuo's text-based waveform pass
• The ast caching caches the value it gets from the globals/closure but it really shouldn't do so because two instances of the same class can have different closure content based on the parameter. I changed ast caching to cache the variable name and look them up on a per-instance basis during elaboration.