WIP - Change materialized relations to be mutable

This branch will change materialized relations to be mutable

* [ ] Change `Axiom::Relation::Materialized#insert` to mutate the internal tuples.
* [ ] Change `Axiom::Relation::Materialized#delete` to mutate the internal tuples.
* [ ] Figure out how to handle inserts on Axiom::Relation::Empty, should it return a new Axiom::Relation::Materialized instance with the new tuples, or should it raise an exception and disallow inserts.
* [ ] Ready for review