This refactors the front-end to use a tree for capturing errors, instead
of throwing exceptions.

This has the benefit that you will receive notifications about all of
the error messages you have, instead of only the first.

I'm a bit fuzzy on the details, since it's been a while since I wrote
these changes.

The main refactoring here is removing the type field from the various
classes, but there are a few other smaller ones.

This also contains the grounds for a refactoring on variable use

Templates should now work, though there is no syntax to reference them
from rules yet

In addition, several internal things have been changed so as to improve
code quality

Simple/expanding variables are now marked with a $, as they are when
used.

Additionally, some debugging help was added

This controls the number of times a rule can be entered into during a
single generations, and is set to 5 by default

RuleCase now handles its own generation.