xo-reader: refactor: move on_colon() for def-expr to define_xs.cpp

include/xo/reader/define_xs.hpp

@@ -38,6 +38,7 @@ namespace xo {
             virtual void on_typedescr(TypeDescr td,
                                       exprstatestack * /*p_stack*/,
                                       rp<Expression> * /*p_emit_expr*/) override;
+            virtual void on_colon(exprstatestack * p_stack) override;
 
         private:
             /**