function callRule (rule, value, siblingState, instance) {
  return rule.call(instance, unwrap(value), unwrap(siblingState), instance)
}
Calls a validation rule by unwrapping its value first from a ref.
@param {Validator} rule
@param {Ref} value
@param {VueInstance} instance
@param {Object} siblingState
@return {Promise<ValidatorResponse> | ValidatorResponse}