Significant parser cleanup:

Significant parser cleanup:
- commented public interface
- much better and very precise error messages
- much better tracing output
- many more checks (still permits more than just syntactically legal
  programs, but much more is checked that can be checked w/o semantic information)
- updated with respect to updated AST
- general cleanup throughout

Parser almost ready for move into lib/go.

R=r
OCL=26853
CL=26855