Destructors: move into nkTupleConstr and move on tuple unpacking (#9776)

tests/destructor/t6434.nim

@@ -1,21 +1,26 @@
 discard """
   exitcode: 0
-  output: '''assingment
-assingment
-assingment
-assingment
-'''
+  output: ""
 """
 
 type
   Foo* = object
     boo: int
 
+var sink_counter = 0
+var assign_counter = 0
+
+proc `=sink`(dest: var Foo, src: Foo) =
+  sink_counter.inc
+
 proc `=`(dest: var Foo, src: Foo) =
-  debugEcho "assingment"
+  assign_counter.inc
 
 proc test(): auto =
   var a,b : Foo
-  return (a, b)
+  return (a, b, Foo(boo: 5))
 
-var (a, b) = test()
+var (a, b, _) = test()
+
+doAssert: assign_counter == 0
+doAssert: sink_counter == 9