Simple prefix changes working; 52.water works.

Upcoming items:
* Drop '.-mode_name' grammar once we have mode-start prefix changes.
  This will include some things marked as a TODO inline in this commit.
* It's probably better to have layer{1,2}.water in one file after all
  because, e.g., changing a rule that's mutually referred to across the
  files is currently very tricky to compile into a new base_grammar
  file. I'll have to think carefully about how to deal with the newlines
  issue.