ExtensionArray.argsort and nargsort
47ffcd674 changes how we handle sorting EAs. We've made `nargsort` compatible with extension arrays. It now calls `ExtensionArray._values_for_argsort` and then applies pandas `nargsort` sorting algorithm to the values.

This may have broken the (implied) ability for ExtensionArray's to "bring their own" sorting algorithm. For example, you can imagine a GPU-backed EA that has a very fast sorting algorithm available. While `ExtensionArray.argsort` can use that custom sorting algo, pandas *may* not. Anywhere that uses `nargsort(obj)` will now essentially do `nargsort(thing._values_for_argsort())` for EAs.