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/semexprs.nim

@@ -1601,7 +1601,7 @@ proc borrowCheck(c: PContext, n, le, ri: PNode) =
   proc scopedLifetime(c: PContext; ri: PNode): bool {.inline.} =
     let n = getRoot(ri, followDeref = false)
     result = (ri.kind in nkCallKinds+{nkObjConstr}) or
-      (n.kind == nkSym and n.sym.owner == c.p.owner)
+      (n.kind == nkSym and n.sym.owner == c.p.owner and n.sym.kind != skResult)
 
   proc escapes(c: PContext; le: PNode): bool {.inline.} =
     # param[].foo[] = self  definitely escapes, we don't need to