Bring back __inner_8 and friends in base_grammar.

I'm not sure why this was missing, actually.

I still haven't figured out why 51.water is broken :(

That's what I'm working on.
Fix 51.water by putting _end_parse in Rule.

_end_parse used to only be in SeqRule, but since OrRule can now handle
atomic items, there are many common cases to handle upon parsing. The
missing element for 51.water used to be that substitutions weren't
handled correctly.

Next up: continue toward a+*b syntax.