ARC: cycle detector (#12823)

* first implementation of the =trace and =dispose hooks for the cycle collector
* a cycle collector for ARC: progress
* manual: the .acyclic pragma is a thing once again
* gcbench: adaptations for --gc:arc
* enable valgrind tests for the strutils tests
* testament: better valgrind support
* ARC refactoring: growable jumpstacks
* ARC cycle detector: non-recursive algorithm
* moved and renamed core/ files back to system/
* refactoring: --gc:arc vs --gc:orc since 'orc' is even more experimental and we want to ship --gc:arc soonish

tests/destructor/tcycle1.nim

@@ -0,0 +1,53 @@
+discard """
+  output: "MEM 0"
+  cmd: "nim c --gc:orc $file"
+"""
+
+type
+  Node = ref object of RootObj
+    le, ri: Node
+    name: char
+
+proc edge(a, b: Node) =
+  if a.le == nil: a.le = b
+  else: a.ri = b
+
+proc createNode(name: char): Node =
+  new result
+  result.name = name
+
+#[
+
++---------+      +------+
+|         |      |      |
+|  A      +----->+      <------+-------------+
++--+------+      |      |      |             |
+   |             |      |      |     C       |
+   |             |  R   |      |             |
++--v------+      |      |      +-------------+
+|         |      |      |        ^
+|   B     <------+      |        |
+|         |      |      +--------+
++---------+      |      |
+                 +------+
+
+]#
+
+proc main =
+  let a = createNode('A')
+  let b = createNode('B')
+  let r = createNode('R')
+  let c = createNode('C')
+
+  a.edge b
+  a.edge r
+
+  r.edge b
+  r.edge c
+
+  c.edge r
+
+
+let mem = getOccupiedMem()
+main()
+echo "MEM ", getOccupiedMem() - mem