It would be nice to make it impossible for other crates to implement our *Ext traits, because then we could freely add new methods without breaking changes. (Adding the indexed_fold_skipnan method to MaybeNanExt in #33 is an example. If ndarray-stats was the only crate that could implement MaybeNanExt, then we could add indexed_fold_skipnan without that being a breaking change.)

ndarray accomplishes this for some of its traits (e.g. the Dimension trait) using a private marker type.