Less naive rule reversal

[vvvvalvalval]

The current implementation of datalog-rules/reversed is pretty naive, as it only consists of a shallow reversal of the clauses in the body of the rules.

This strategy fails to meet its performance goals as soon as the body of the rule contains nested conjunctions of clauses, e.g in an or-join or not-join. Deeply reversing the body of such clauses may be a better approach.