Potential increase in performance via symmetry of Hessian

[adrhill]

The Hessian tracer added in #20 keeps track of all first and second-order interactions between indices. However, these aren't all needed due to symmetry.
To make an analogy using Polynomials, we separately keep track of both $x_1x_2$ and $x_2x_1$.

By adding numerical comparisons between indices before updating sets, we could potentially speed up performance on some set types outside of BitSets.

It would be good to add a benchmarking suite with multiple set types before working on this.