proof that refcounting can handle Nim's async (#12533)

lib/system/gc.nim

@@ -16,7 +16,8 @@
 
 const
   CycleIncrease = 2 # is a multiplicative increase
-  InitialCycleThreshold = 4*1024*1024 # X MB because cycle checking is slow
+  InitialCycleThreshold = when defined(nimCycleBreaker): high(int)
+                          else: 4*1024*1024 # X MB because cycle checking is slow
   InitialZctThreshold = 500  # we collect garbage if the ZCT's size
                              # reaches this threshold
                              # this seems to be a good value
@@ -792,6 +793,12 @@ proc collectCT(gch: var GcHeap) =
     collectCTBody(gch)
     gch.zctThreshold = max(InitialZctThreshold, gch.zct.len * CycleIncrease)
 
+proc GC_collectZct*() =
+  ## Collect the ZCT (zero count table). Unstable, experimental API for
+  ## testing purposes.
+  ## DO NOT USE!
+  collectCTBody(gch)
+
 when withRealTime:
   proc toNano(x: int): Nanos {.inline.} =
     result = x * 1000