Make more use of RoleSpec struct

Most code was casting this through a generic Node.  By declaring
everything as RoleSpec appropriately, we can remove a bunch of casts and
ad-hoc node type checking.

Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>

src/backend/commands/policy.c

@@ -177,7 +177,7 @@ policy_role_list_to_array(List *roles, int *num_roles)
 		}
 		else
 			role_oids[i++] =
-				ObjectIdGetDatum(get_rolespec_oid((Node *) spec, false));
+				ObjectIdGetDatum(get_rolespec_oid(spec, false));
 	}
 
 	return role_oids;