1. Add references and start citing them.

2. Profile (and fix?) any remaining slow operations.

3. When we take a Cartesian product involving a trivial algebra, we
   could easily cache the identity and charpoly coefficients using
   the nontrivial factor. On the other hand, it's nice that we can
   test out some alternate code paths...

4. Add dimension bounds on any tests over AA that compute element
   subalgebras.

5. The rational_algebra() stuff doesn't really belong in classes that
   don't derive from RationalBasisEJA or its as-yet-nonexistent
   element class.