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

testament/testament.nim

@@ -449,7 +449,7 @@ proc testSpecHelper(r: var TResults, test: TTest, expected: TSpec, target: TTarg
             else:
               exeCmd = exeFile
             if expected.useValgrind:
-              args = exeCmd & args
+              args = @["--error-exitcode=1"] & exeCmd & args
               exeCmd = "valgrind"
           var (_, buf, exitCode) = execCmdEx2(exeCmd, args, input = expected.input)
           # Treat all failure codes from nodejs as 1. Older versions of nodejs used