fixes #13781; fixes #14901; add acyclic pragmas to FlowVar (#20804)

* add acyclic pragmas to FlowVar

* add testcases

lib/pure/concurrency/threadpool.nim

@@ -102,7 +102,7 @@ type
     idx: int
 
   FlowVarBase* = ref FlowVarBaseObj ## Untyped base class for `FlowVar[T] <#FlowVar>`_.
-  FlowVarBaseObj = object of RootObj
+  FlowVarBaseObj {.acyclic.} = object of RootObj
     ready, usesSemaphore, awaited: bool
     cv: Semaphore  # for 'blockUntilAny' support
     ai: ptr AwaitInfo
@@ -111,7 +111,7 @@ type
                    # be RootRef here otherwise the wrong GC keeps track of it!
     owner: pointer # ptr Worker
 
-  FlowVarObj[T] = object of FlowVarBaseObj
+  FlowVarObj[T] {.acyclic.} = object of FlowVarBaseObj
     blob: T
 
   FlowVar*[T] {.compilerproc.} = ref FlowVarObj[T] ## A data flow variable.

tests/parallel/tlet_spawn.nim

@@ -4,7 +4,7 @@ done999 999
 '''
 """
 
-import threadpool
+import std/[threadpool, os]
 
 proc foo(): int = 999
 
@@ -17,3 +17,12 @@ proc main =
   echo "done", f, " ", b
 
 main()
+
+# bug #13781
+proc thread(): string =
+  os.sleep(1000)
+  return "ok"
+
+var fv = spawn thread()
+sync()
+doAssert ^fv == "ok"