lift the =dup hook (#21903)

* fixes tests again
* remove helper functions
* fixes closures, owned refs
* final cleanup

lib/system/arc.nim

@@ -200,10 +200,6 @@ proc nimDecRefIsLast(p: pointer): bool {.compilerRtl, inl.} =
       when traceCollector:
         cprintf("[DECREF] %p\n", cell)
 
-proc nimDupRef(dest: ptr pointer, src: pointer) {.compilerRtl, inl.} =
-  dest[] = src
-  if src != nil: nimIncRef src
-
 proc GC_unref*[T](x: ref T) =
   ## New runtime only supports this operation for 'ref T'.
   var y {.cursor.} = x