fixes the recently discovered GC memory leaks

This revision is intended as comparison point between the old and the new GC
The used GC can be switched in mmdisp and various statistics will be gathered during
execution (these will be removed/disabled in later revisions)

tests/gc/refarrayleak.nim

@@ -0,0 +1,39 @@
+discard """
+  outputsub: "no leak: "
+"""
+
+type
+  TNode = object
+    data: array[0..300, char]
+
+  PNode = ref TNode
+
+  TNodeArray = array[0..10, PNode]
+
+  TArrayHolder = object
+    sons: TNodeArray
+
+proc nullify(a: var TNodeArray) =
+  for i in 0..high(a):
+    a[i] = nil
+
+proc newArrayHolder: ref TArrayHolder =
+  new result
+
+  for i in 0..high(result.sons):
+    new result.sons[i]
+
+  nullify result.sons
+
+proc loop =
+  for i in 0..10000:
+    discard newArrayHolder()
+    
+  if getOccupiedMem() > 300_000:
+    echo "still a leak! ", getOccupiedMem()
+    quit 1
+  else:
+    echo "no leak: ", getOccupiedMem()
+
+loop()
+