Let's compute gb of the ideal of a quotient ring lazily!  When it's time to
reduce a ring element to normal form, then compute it.  And if it's
homogeneous, compute it just up to the required degree.

This will require having ring elements visible at top level that are not yet in
normal form, but would be reduced automatically internally.  

Note: the hash code of a ring element needs to be computed from the normal
form.
