Dynamic properties are not checked for consistency with parent

[hadley]

foo := new_class(properties = list(x = class_integer))
foo2 := new_class(
  parent = foo,
  properties = list(
    x = new_property(
      class_character,
      getter = function(self) self@x,
      setter = function(self, value) {
        self@x <- value
        self
      }
    )
  )
)

This seems like a violation of #352, but was deliberately carved out of #691: https://github.com/RConsortium/S7/pull/691/changes#diff-db883c7d4a79cfb06f9b46be422e9619820b841e80f70a2592cad16653adebe6R444-R448