destructors: preparations for upcoming changes

lib/system.nim

@@ -292,6 +292,13 @@ when defined(nimArrIdx):
   proc `[]=`*[I: Ordinal;T,S](a: T; i: I;
     x: S) {.noSideEffect, magic: "ArrPut".}
   proc `=`*[T](dest: var T; src: T) {.noSideEffect, magic: "Asgn".}
+  when defined(nimNewRuntime):
+    template `=destroy`*[T](x: var T) =
+      ## generic `destructor`:idx: implementation that can be overriden.
+      discard
+    template `=sink`*[T](x: var T; y: T) =
+      ## generic `sink`:idx: implementation that can be overriden.
+      shallowCopy(x, y)
 
 type
   Slice*[T] = object ## builtin slice type