xo-reader: refactor: move def on_expr() to define_xs subtype

src/reader/exprstate.cpp

@@ -0,0 +1,15 @@
+/* @file exprstate.cpp */
+
+#include "exprstate.hpp"
+#include "define_xs.hpp"
+
+namespace xo {
+    namespace scm {
+        std::unique_ptr<define_xs>
+        define_xs::def_0(rp<DefineExprAccess> def_expr) {
+            return std::make_unique<define_xs>(define_xs(def_expr));
+        }
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end exprstate.cpp */