Add Validation

[dyoder]

We can potentially do some validation prior to running a ruleset to help rulebase creators detect errors. There are at least two checks we can perform:

1. We can detect unused actions or conditions by doing a traversal of the rule graph and checking that against the action and condition dictionaries.

2. We can detect condition closures that can never evaluate to true by looking for the presence of both a condition and its negation.