Work toward layer{1,2}/base_grammar alignment.

To keep the current rule definition setup, I will need to push new modes
*after* mode_params is called. This is temporarily breaking other
things. I can't remember why I made the decision to push before calling
mode_params, so I'm afraid I may have to rediscover that the hard way.
In the meantime, I'll see if I can get things working starting from
parse.pending.py.