add opcode for swapping top two values on the stack

src/rdi/rdi.mdesk

@@ -1349,7 +1349,8 @@ RDI_EvalOpTable:
   {CallSiteValue   48      4 0 0}
   {PartialValue    49      4 0 0}
   {PartialValueBit 50      8 0 0}
-  {COUNT           51      0 0 0}
+  {Swap            51      0 2 2} // swaps the top two entries on the stack
+  {COUNT           52      0 0 0}
 }
 
 // NOTE(rjf): "ck" -> "conversion kind, when converted to type group", used in square matrix form