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

doc/manual.rst

@@ -6017,8 +6017,32 @@ The ``noreturn`` pragma is used to mark a proc that never returns.
 
 acyclic pragma
 --------------
-The ``acyclic`` pragma applies to type declarations. It is deprecated and
-ignored.
+The ``acyclic`` pragma can be used for object types to mark them as acyclic
+even though they seem to be cyclic. This is an **optimization** for the garbage
+collector to not consider objects of this type as part of a cycle:
+
+.. code-block:: nim
+  type
+    Node = ref NodeObj
+    NodeObj {.acyclic.} = object
+      left, right: Node
+      data: string
+
+Or if we directly use a ref object:
+
+.. code-block:: nim
+  type
+    Node {.acyclic.} = ref object
+      left, right: Node
+      data: string
+
+In the example a tree structure is declared with the ``Node`` type. Note that
+the type definition is recursive and the GC has to assume that objects of
+this type may form a cyclic graph. The ``acyclic`` pragma passes the
+information that this cannot happen to the GC. If the programmer uses the
+``acyclic`` pragma for data types that are in reality cyclic, the memory leaks
+can be the result, but memory safety is preserved.
+
 
 
 final pragma