Don't generate anchors at comptime when dumping

* this is a prerequisite to getting dumping to work,
   but dumping is still blocked by #138

yaml/native.nim

@@ -1616,6 +1616,8 @@ proc representChild*[O](
   value: ref O,
 ) =
   if isNil(value): ctx.put(scalarEvent("~", yTagNull))
+  else:
+    when nimvm: discard
     else:
       let p = cast[pointer](value)
       # when c.anchorStyle == asNone, `referenced` is used as indicator that we are
@@ -1654,6 +1656,8 @@ proc representChild*[O](
         ctx.putImpl = origPut
         ctx.put(ex)
     ctx.representChild(value[])
+    when nimvm: discard
+    else:
       if ctx.options.anchorStyle == asNone: ctx.refs[p] = (val.a, false)
 
 proc representChild*[T](