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/tnewruntime_misc.nim

@@ -11,7 +11,7 @@ test
 '''
 """
 
-import core / allocators
+import system / allocators
 import system / ansi_c
 
 import tables
@@ -132,5 +132,5 @@ proc xx(xml: string): MyObject =
   result.x  = xml
   defer: echo stream
 
- 
+
 discard xx("test")