add EquateString to cmpopts about go-cmp

[dsnet]

Thanks for the proposal.

Comparing on the String method is generally considered poor practice since many types do not have documented guarantees of what the String method even means. For example, regexp.Regexp.String does specify that the String method returns source regexp, while reflect.Type.String warns against comparing on the String method. Most types follow the later model where String is intended as human debugging aid and not as a mechanism for semantic interpretation.

Also, a helper function like this will likely overly match on too many types since many types have a String method, leading to this helper option having a high probability of conflicting with another option.