Module todo

[x] The following should get implemented:

[x] Better support for relative extensions.

[ ] Much faster basic arithmetic based on FLINT.

[x] Orders

[ ] Finish relative ideals.

[ ] Fix cyclotomic conversions.

[ ] morphism.py -- need to change it so morphisms of relative number fields
    are specified in a sensible manner.  This will mean giving the image
    of all the gens.  One *shortcut* will be to give morphism from the
    base field.