Fix aconjunctive fragment for rules with >1 children
Previously, the algorithm for the aconjunctive fragment would insert
many cores for each child of a modal rule and add those as children of
the rule application to the state.
This was wrong for multiple reasons:
1. Dependency tracking for the backjumping implementation assumes that
the number of children of a rule application is fixed
2. Propagation with multiple children computes an OR of the children
instead of AND
Now, for each modal rule application multiple rules are added as
children of the state, each with the cores of the original rule paired
with every possible combination of deferrals.