.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

tests/destructor/tlists.nim

@@ -0,0 +1,38 @@
+discard """
+  output: '''Success
+@["a", "b", "c"]
+0'''
+  cmd: '''nim c --gc:destructors $file'''
+"""
+
+import os
+import math
+import lists
+import strutils
+
+proc mkleak() =
+  # allocate 1 MB via linked lists
+  let numberOfLists = 100
+  for i in countUp(1, numberOfLists):
+    var leakList = initDoublyLinkedList[string]()
+    let numberOfLeaks = 5000
+    for j in countUp(1, numberOfLeaks):
+      leakList.append(newString(200))
+
+proc mkManyLeaks() =
+  for i in 0..0:
+    mkleak()
+  echo "Success"
+
+iterator foobar(c: string): seq[string] {.closure.} =
+  yield @["a", "b", c]
+
+proc tsimpleClosureIterator =
+  var myc = "c"
+  for it in foobar(myc):
+    echo it
+
+let startMem = getOccupiedMem()
+mkManyLeaks()
+tsimpleClosureIterator()
+echo getOccupiedMem() - startMem