More consistent file names for coPset, bset and set.

Similar files (gmap, listset, ...) were already in singular form and
matched the name of the set/map data type.

theories/gmap.v

@@ -3,7 +3,7 @@
 (** This file implements finite maps and finite sets with keys of any countable
 type. The implementation is based on [Pmap]s, radix-2 search trees. *)
 From stdpp Require Export countable fin_maps fin_map_dom.
-From stdpp Require Import pmap mapset sets.
+From stdpp Require Import pmap mapset set.
 
 (** * The data structure *)
 (** We pack a [Pmap] together with a proof that ensures that all keys correspond