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/tarc2.nim

@@ -1,6 +1,6 @@
 discard """
-  output: '''leak: true'''
-  cmd: '''nim c --gc:arc $file'''
+  output: '''leak: false'''
+  cmd: '''nim c --gc:orc $file'''
 """
 
 type
@@ -19,11 +19,8 @@ proc addX(x: T; child: T) =
 proc main(rootName: string) =
   var root = create()
   root.data = rootName
-  # this implies we do the refcounting wrong. We should leak memory here
-  # and not create a destruction cycle:
   root.addX root
 
 let mem = getOccupiedMem()
 main("yeah")
-# since we created a retain cycle, we MUST leak memory here:
 echo "leak: ", getOccupiedMem() - mem > 0