.cursor implementation (#12637)

* cursors: first implementation
* added currently failing test
* .cursor works for doubly linked lists
* make -d:useMalloc work again
* added code to nil out refs in a destructor
* it's now called --gc:arc
* renderer.nim: render nkBreakState properly
* make simple closure iterators work without leaking

lib/pure/collections/lists.nim

@@ -76,13 +76,16 @@
 when not defined(nimhygiene):
   {.pragma: dirty.}
 
+when not defined(nimHasCursor):
+  {.pragma: cursor.}
+
 type
   DoublyLinkedNodeObj*[T] = object ## \
     ## A node a doubly linked list consists of.
     ##
     ## It consists of a `value` field, and pointers to `next` and `prev`.
     next*: <//>(ref DoublyLinkedNodeObj[T])
-    prev*: ref DoublyLinkedNodeObj[T]
+    prev* {.cursor.}: ref DoublyLinkedNodeObj[T]
     value*: T
   DoublyLinkedNode*[T] = ref DoublyLinkedNodeObj[T]
 
@@ -100,7 +103,7 @@ type
     ## Use `initSinglyLinkedList proc <#initSinglyLinkedList>`_ to create
     ## a new empty list.
     head*: <//>(SinglyLinkedNode[T])
-    tail*: SinglyLinkedNode[T]
+    tail* {.cursor.}: SinglyLinkedNode[T]
 
   DoublyLinkedList*[T] = object ## \
     ## A doubly linked list.
@@ -108,7 +111,7 @@ type
     ## Use `initDoublyLinkedList proc <#initDoublyLinkedList>`_ to create
     ## a new empty list.
     head*: <//>(DoublyLinkedNode[T])
-    tail*: DoublyLinkedNode[T]
+    tail* {.cursor.}: DoublyLinkedNode[T]
 
   SinglyLinkedRing*[T] = object ## \
     ## A singly linked ring.
@@ -116,7 +119,7 @@ type
     ## Use `initSinglyLinkedRing proc <#initSinglyLinkedRing>`_ to create
     ## a new empty ring.
     head*: <//>(SinglyLinkedNode[T])
-    tail*: SinglyLinkedNode[T]
+    tail* {.cursor.}: SinglyLinkedNode[T]
 
   DoublyLinkedRing*[T] = object ## \
     ## A doubly linked ring.