Make action descriptions ubiquitous

Currently, action descriptions (used for the simulate command) may only be added to a special kind of policy (i.e. with --kind 'action' on the CLI). It may be possible to include action descriptions in any policy, and it would eliminate some confusion it that were possible. This would require at least two changes.

1. Relaxing the syntax checking of NonrecurisveRuleTheory so that action-descriptions can be added (e.g. even if they reference other policies in the head of the rule). We'd only want to do this for the action descriptions themselves--not the regular rules used to declare policy.

2. Modifying the simulate() functionality to properly modify the current policy in case the head of the action-description rule does not specify which policy to make the changes to.

We would want to make sure that despite relaxing (1) we don't allow the user to write policies that cause undesirable behavior in the policy engine (e.g. infinite loops).