.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

compiler/renderer.nim

@@ -1526,6 +1526,8 @@ proc gsub(g: var TSrcGen, n: PNode, c: TContext) =
 
   of nkBreakState:
     put(g, tkTuple, "breakstate")
+    if renderIds in g.flags:
+      gsons(g, n, c, 0)
   of nkTypeClassTy:
     gTypeClassTy(g, n)
   else: