chore(field_theory/ratfunc): comm_ring (ratfunc K) (#11188)
Previously, the file only gave a `field (ratfunc K)` instance,
requiring `comm_ring K` and `is_domain K`.
In fact, `ratfunc K` is a `comm_ring` regardless of the `is_domain`.
The upstream instance is proven first, with a generalized tactic.
