fixes withRegion for --gc:stack

lib/system/alloc.nim

@@ -9,7 +9,6 @@
 
 # Low level allocator for Nim. Has been designed to support the GC.
 # TODO:
-# - eliminate "used" field
 # - make searching for block O(1)
 {.push profiler:off.}
 

lib/system/gc_stack.nim

@@ -79,7 +79,7 @@ template withRegion*(r: MemRegion; body: untyped) =
   try:
     body
   finally:
-    r = tlRegion
+    #r = tlRegion
     tlRegion = oldRegion
 
 template inc(p: pointer, s: int) =