Add some missing *Labels functions

Add `ArrayLabels.make_float` (0093f22), corresponding to `Array.make_float` and `ListLabels.sort_uniq` (bbfacf0), corresponding to `List.sort_uniq`.

Also make the documentation consistent, changing `List.` to `ListLabels.` (f7ae29a) and `Array.` to `ArrayLabels.` (87a4542) where appropriate (i.e. except in descriptions of raised exceptions) and similar