A trait representing a type with a property used to be fetched, and a route with which to
fetch it. This can be either a tag or the limit of fetching, for example.
A trait representing a weaker variant of GetFetchProp; only indicates the fetching route can
be obtained by the get_route method by specifying a property, not that such property is
obtainable. Note that all GetFetchProp implementers also implement PropRouteable, thanks
to a blanket impl.