xo-reader: + parserstatemachine.push_exprstate(); simplify lambda_xs

src/reader/parserstatemachine.cpp

@@ -12,6 +12,16 @@ namespace xo {
         parserstatemachine::pop_exprstate() {
             return p_stack_->pop_exprstate();
         }
+
+        exprstate &
+        parserstatemachine::top_exprstate() {
+            return p_stack_->top_exprstate();
+        }
+
+        void
+        parserstatemachine::push_exprstate(std::unique_ptr<exprstate> x) {
+            p_stack_->push_exprstate(std::move(x));
+        }
     } /*namespace scm*/
 } /*namespace xo*/