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/tcycle3.nim

@@ -0,0 +1,64 @@
+discard """
+  output: '''BEGIN
+END
+END 2
+0'''
+  cmd: '''nim c --gc:orc $file'''
+"""
+
+# extracted from thavlak.nim
+
+type
+  BasicBlock = ref object
+    inEdges: seq[BasicBlock]
+    outEdges: seq[BasicBlock]
+    name: int
+
+proc newBasicBlock(name: int): BasicBlock =
+  result = BasicBlock(
+    inEdges: newSeq[BasicBlock](),
+    outEdges: newSeq[BasicBlock](),
+    name: name
+  )
+
+type
+  Cfg = object
+    basicBlockMap: seq[BasicBlock]
+    startNode: BasicBlock
+
+proc newCfg(): Cfg =
+  result = Cfg(
+    basicBlockMap: newSeq[BasicBlock](),
+    startNode: nil)
+
+proc createNode(cfg: var Cfg, name: int): BasicBlock =
+  if name < cfg.basicBlockMap.len:
+    result = cfg.basicBlockMap[name]
+  else:
+    result = newBasicBlock(name)
+    cfg.basicBlockMap.setLen name+1
+    cfg.basicBlockMap[name] = result
+
+proc newBasicBlockEdge(cfg: var Cfg, fromName, toName: int) =
+  echo "BEGIN"
+  let fr = cfg.createNode(fromName)
+  let to = cfg.createNode(toName)
+
+  fr.outEdges.add(to)
+  to.inEdges.add(fr)
+
+proc run(cfg: var Cfg) =
+  cfg.startNode = cfg.createNode(0) # RC = 2
+  newBasicBlockEdge(cfg, 0, 1) #
+  echo "END"
+
+  discard cfg.createNode(1)
+
+proc main =
+  var c = newCfg()
+  c.run
+  echo "END 2"
+
+let mem = getOccupiedMem()
+main()
+echo getOccupiedMem() - mem