classes: change class-usage to return a sequence like it's documented to do.

core/classes/classes.factor

@@ -131,7 +131,7 @@ GENERIC: implementors ( class/classes -- seq )
         tri
     ] { } make ;
 
-: class-usage ( class -- seq ) update-map get at ;
+: class-usage ( class -- seq ) update-map get at keys ;
 
 <PRIVATE
 
@@ -146,7 +146,7 @@ GENERIC: implementors ( class/classes -- seq )
 PRIVATE>
 
 : class-usages ( class -- seq )
-    [ class-usage keys ] closure sets:members ;
+    [ class-usage ] closure sets:members ;
 
 M: class implementors implementors-map get at sets:members ;
 
@@ -184,7 +184,7 @@ GENERIC: metaclass-changed ( use class -- )
 : check-metaclass ( class metaclass -- usages/f )
     over class? [
         over "metaclass" word-prop eq?
-        [ drop f ] [ class-usage keys ] if
+        [ drop f ] [ class-usage ] if
     ] [ 2drop f ] if ;
 
 : ?define-symbol ( word -- )

core/classes/tuple/tuple.factor

@@ -298,7 +298,7 @@ PRIVATE>
 GENERIC: make-final ( class -- )
 
 M: tuple-class make-final
-    [ dup class-usage keys ?metaclass-changed ]
+    [ dup class-usage ?metaclass-changed ]
     [ t "final" set-word-prop ]
     bi ;