feat(algebra/order/sub): add `canonically_ordered_add_monoid.to_add_cancel_comm_monoid` (#15955)
feat(algebra/order/sub): add `canonically_ordered_add_monoid.to_add_cancel_comm_monoid` (#15955)

We don't have a typeclass for cancellative ordered additive monoids (such as `nat` or finsupps with nat as their codomain).
This definition is enough to create the instance mid-proof if needed, so is better than nothing.

We already have similar definitions such as `no_zero_divisors.to_cancel_comm_monoid` for rings.





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