Existential rules

[quoll]

This allows for entirely new entities to be created as the output of a rule. This will need:

1. One unbound variable permitted in the entity position (both in the Pabu grammar and in the rule definitions).
2. Rules with unbound vars in the head will need different execution for insert/select. Possibly requiring a select that leads to data that gets transformed before being sent to an insert (which is likely how it happens now).
3. Rewriting rules to subtract all statements that would lead to the current head from the results, so new entities are not created for the multiple runs of a rule.

Depends on #47 for subtractions, and #48 for multiple predicates in a rule.