1. Add CartesianProductEJA.

2. Add references and start citing them.

3. Implement the octonion simple EJA.

4. Override random_instance(), one(), et cetera in DirectSumEJA.

5. Switch to QQ in *all* algebras for _charpoly_coefficients().
   This only works when we know that the basis can be rationalized...
   which is the case at least for the concrete EJAs we provide,
   but not in general.

6. Pass already_echelonized (default: False) and echelon_basis
   (default: None) into the subalgebra constructor. The value of
   already_echelonized can be passed to V.span_of_basis() to save
   some time, and usinf e.g. FreeModule_submodule_with_basis_field
   we may somehow be able to pass the echelon basis straight in to
   save time.

   This may require supporting "basis" as a list of basis vectors
   (as opposed to superalgebra elements) in the subalgebra constructor.

7. Use charpoly for inverse stuff if it's cached.