turn destructors into finalizers

tests/destructor/turn_destroy_into_finalizer.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''true'''
+"""
+
+type
+  Foo = object
+    id: int
+
+var destroyed: int
+
+proc `=destroy`(x: var Foo) =
+  #echo "finally ", x.id
+  inc destroyed
+
+proc main =
+  var r: ref Foo
+  for i in 1..50_000:
+    new(r)
+    r.id = i
+  echo destroyed > 30_000
+
+main()