consumes ASTs that follow the Mozilla Parser API spec:
https://developer.mozilla.org/en-US/docs/SpiderMonkey/Parser_API
passes all tests on OS X; performance is comparable to previous
version. also includes some miscellaneous cleanup.
remaining issues:
- only Rhino AST builder is supported
- node visitors (old and new) may not be hooked up yet
- circular-reference issues in doclets
- docs are (mostly) missing
- various other TODO comments
