`ring` doesn't support negation

[digama0]

Currently the implementation of ring is incomplete and has no understanding of subtraction or negation:

import Mathlib.Tactic.Ring

example (a b : ℤ) : a - b = a + -b := by ring
-- ring failed, ring expressions not equal: 
-- (1) * (a - b)^1 + 0
--   !=
-- (1) * (a)^1 + (1) * (-b)^1 + 0

[0art0]

I suspect that this may be because of this line in the code. Right now, it seems conditional on the implementation of rational numbers in Mathlib.