*   Add `deleteMaybe` for `NEMap`, `NEIntMap`, `NESet`, and `NEIntSet`.
*   Add indexed traversal instances for `NEMap`, `NEIntMap`, and `NESeq`.
*   Add `IsNonEmptyList` for uniform non-empty list conversions.
*   Add union functions for combining possibly-empty and non-empty maps and sets.
*   Fix `NESeq.adjust'` to use the strict `Seq.adjust'` in the tail.