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/tcycle2.nim

@@ -0,0 +1,18 @@
+discard """
+  output: "MEM 0"
+  cmd: "nim c --gc:orc $file"
+"""
+
+type
+  Node = ref object
+    kids: seq[Node]
+    data: string
+
+proc main(x: int) =
+  var n = Node(kids: @[], data: "3" & $x)
+  let m = n
+  n.kids.add m
+
+let mem = getOccupiedMem()
+main(90)
+echo "MEM ", getOccupiedMem() - mem