Allow GOOPS getters to add methods to primitive generics

Allow GOOPS getters to add methods to primitive generics

* module/oop/goops.scm (ensure-generic): If the old definition of a
  desired getter is a primitive generic, let the new method be added to
  it instead of creating a fresh new generic.

  (ensure-accessor): Modify as necessary to keep the old behavior.
  Maybe something more optimal can be done here, but it's not yet
  obvious to me how to do it.