This makes the two maps in GenerationState private. Use the
defineVar/findVar or defineRuleVar/findRuleVar instead.

This is another change on the way to doing autovivify

A major refactoring to the internals of how things work. I believe that
nothing is broken

There is now syntax for rule references. Use @ instead of $, and use :=
to indicate you want exhaustion enabled

Use @ to refer to it

Case elements are now responsible for generating themselves.