xo-reader: simplify: adopt exprseq_xs + drop .admits_definition()

include/xo/reader/exprseq_xs.hpp

@@ -15,6 +15,11 @@ namespace xo {
          *
          **/
         class exprseq_xs : public exprstate {
+        public:
+            exprseq_xs();
+
+            static std::unique_ptr<exprseq_xs> expect_toplevel_expression_sequence();
+
         public:
             // ----- token input methods -----