ARC: fixes cycle detection and move the .cursor attribute into closures (#12872)

tests/destructor/tasync_prototype.nim

@@ -0,0 +1,54 @@
+discard """
+  output: '''asdas
+processClient end
+false
+MEMORY 0
+'''
+  cmd: '''nim c --gc:arc $file'''
+"""
+
+type
+  PAsyncHttpServer = ref object
+    value: string
+  PFutureBase {.acyclic.} = ref object
+    callback: proc () {.closure.}
+    value: string
+    failed: bool
+
+proc accept(server: PAsyncHttpServer): PFutureBase =
+  new(result)
+  result.callback = proc () =
+    discard
+  server.value = "hahaha"
+
+proc processClient(): PFutureBase =
+  new(result)
+
+proc serve(server: PAsyncHttpServer): PFutureBase =
+  iterator serveIter(): PFutureBase {.closure.} =
+    echo server.value
+    while true:
+      var acceptAddrFut = server.accept()
+      yield acceptAddrFut
+      var fut = acceptAddrFut.value
+
+      var f {.cursor.} = processClient()
+      when true:
+        f.callback =
+          proc () =
+            echo("processClient end")
+            echo(f.failed)
+      yield f
+  var x = serveIter
+  for i in 0 .. 1:
+    result = x()
+    if result.callback != nil:
+      result.callback()
+
+let mem = getOccupiedMem()
+
+proc main =
+  discard serve(PAsyncHttpServer(value: "asdas"))
+
+main()
+echo "MEMORY ", getOccupiedMem() - mem