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/tselect.nim

@@ -0,0 +1,26 @@
+discard """
+   output: '''abcsuffix
+xyzsuffix'''
+  cmd: '''nim c --gc:arc $file'''
+"""
+
+proc select(cond: bool; a, b: sink string): string =
+  if cond:
+    result = a # moves a into result
+  else:
+    result = b # moves b into result
+
+proc test(param: string; cond: bool) =
+  var x = "abc" & param
+  var y = "xyz" & param
+
+  # possible self-assignment:
+  x = select(cond, x, y)
+
+  echo x
+  # 'select' must communicate what parameter has been
+  # consumed. We cannot simply generate:
+  # (select(...); wasMoved(x); wasMoved(y))
+
+test("suffix", true)
+test("suffix", false)