feat(tactic/ring): recursive ring_nf (#14429)
As [reported on Zulip](https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/.60ring_nf.60.20not.20consistently.20normalizing.3F). This allows `ring_nf` to rewrite inside the atoms of a ring expression, meaning that things like `f (a + b) + c` can simplify in both `+` expressions.
