nim-lang#55 Remove smart-ptr data-race

Using the memory order "Release" here enables a data-race according to tsan.

The data-race being *potentially* a false-positive.
The race it identifies is between 2 threads both trying to destroy the smartPtr at the same time.

They'll both call the decr proc in smartptrs.nim:92 and get past the nil check. Then one of them might get past the second if check and deallocate the ptr.
However, that should be impossible to lead to a data race.
The count of 0 can only be reached *once* if you're the last thread.
I can only assume compiler-order-reshuffling may enable a race here.

Using the default mode (SeqCst) as well as Acquire Release  (AcqRel) gets rid of said data-race.

threading/smartptrs.nim

@@ -93,7 +93,7 @@ proc decr[T](p: SharedPtr[T]) {.inline.} =
   if p.val != nil:
     # this `fetchSub` returns current val then subs
     # so count == 0 means we're the last
-    if p.val.counter.fetchSub(1, Release) == 0:
+    if p.val.counter.fetchSub(1, AcqRel) == 0:
       `=destroy`(p.val.value)
       deallocShared(p.val)
 
@@ -119,7 +119,7 @@ proc newSharedPtr*[T](val: sink Isolated[T]): SharedPtr[T] {.nodestroy.} =
   ## Returns a shared pointer which shares
   ## ownership of the object by reference counting.
   result.val = cast[typeof(result.val)](allocShared(sizeof(result.val[])))
-  int(result.val.counter) = 0
+  result.val.counter.store(0)
   result.val.value = extract val
 
 template newSharedPtr*[T](val: T): SharedPtr[T] =