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

compiler/lambdalifting.nim

@@ -353,6 +353,7 @@ proc createUpField(c: var DetectionPass; dest, dep: PSym; info: TLineInfo) =
   # with cycles properly, so it's better to produce a weak ref (=ptr) here.
   # This seems to be generally correct but since it's a bit risky it's disabled
   # for now.
+  # XXX This is wrong for the 'hamming' test, so remove this logic again.
   let fieldType = if isDefined(c.graph.config, "nimCycleBreaker"):
                     c.getEnvTypeForOwnerUp(dep, info) #getHiddenParam(dep).typ
                   else: