Add positive_vars(t) and negative_vars(t) in the Subst API
There is no way in the current Types.Subst API to determine, given a variable v and a type t, whether v appears only in a covariant position in t, contravariant position, or both. Adding such a function would be useful, or alternatively (depending on the performance), a function positive_vars(t) that returns all the variables appearing in a covariant position in t, and negative_vars(t) that returns all the variables appearing in a contravariant position in t.