fix(int/basic): change order of instances to int.cast (#877)
As discussed at https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/Problem.20with.20type.20class.20search/near/161848744
Changing the order of arguments lets type class inference fail quickly for `int -> nat` coercions, rather than repeatedly looking for `has_neg` on `nat`.
