I mostly didn't put it into `std::vec` because I don't want to expose `MaybeOwnedVector` as a standard API quite yet.  The proper design for a DST-aware `MaybeOwned` has its own thorny issues (i.e. should it have three variants, or just two?)  i don't feel as bad about providing a potentially flaky API from within the experimental `graphviz` module as I would about providing such a flaky API from `std::vec`.