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/pragmas.nim

@@ -940,7 +940,7 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: var int,
       of wAcyclic:
         noVal(c, it)
         if sym.typ == nil: invalidPragma(c, it)
-        # now: ignored
+        else: incl(sym.typ.flags, tfAcyclic)
       of wShallow:
         noVal(c, it)
         if sym.typ == nil: invalidPragma(c, it)