xo-interpreter2: VSM.start_eval() returns const ref

If copied get unstable value, since may be moved away on next gc

xo-interpreter2/include/xo/interpreter2/vsm/DVirtualSchematikaMachine.hpp

@@ -119,7 +119,7 @@ namespace xo {
             /** evaluate expression @p expr
              *  Require: must first start interactive/batch session
              **/
-            VsmResult start_eval(obj<AExpression> expr);
+            const VsmResult & start_eval(obj<AExpression> expr);
 
             /** borrow calling thread to run indefinitely,
              *  until halt instruction

xo-interpreter2/src/interpreter2/DVirtualSchematikaMachine.cpp

@@ -225,7 +225,7 @@ namespace xo {
             return VsmResultExt(VsmResult(*p_value), remaining);
         }
 
-        VsmResult
+        const VsmResult &
         DVirtualSchematikaMachine::start_eval(obj<AExpression> expr)
         {
             this->pc_ = VsmInstr::c_eval;