Debugger

[olwmc]

This is a preliminary pull request so that other folks can have an easy way to check out the new debugger code.

Engineering/design/* suggestions/comments/* are all very much appreciated and welcome. Luckily, everything is fairly independent and loosely coupled so I can change basically anything without too much difficulty.

It should be noted that I've worked on this in a vacuum for the entire summer. So, there's likely dozens of very low hanging bugs for us to squash 😄.

You can run the debugger with asteroid --adb <FILENAME> (python asteroid/__init__.py --adb <FILENAME>). There are some basic docs in adb/docs/`. I kept them fairly small so that if we make major changes now I can easily change things.

There is an experimental explicit mode that exposes pattern matching. Right now it is very experimental and that should be noted on any release documents. My plan for the semester is to beef it up and improve the debugger overall.

RE: How do these changes affect asteroid development?

For most situations, the inclusion of the debugger does not change the development experience. Following what I've done in other nodes and unify clauses should be pretty straight forward. There's documentation at the bottom of the walk.py file for all of the functions I used to get the debugger's behavior working within the walker.