add public kSlotSize to IndexedMemPool

Summary:
add public kSlotSize to IndexedMemPool,
needed to support getting the approximate memory footprint of the pool
(since a Slot has two atomic uint32_t's more than a bare Elem)

Reviewed By: yfeldblum

Differential Revision: D5690225

fbshipit-source-id: 667da6b67b339038b92b0e5acde17219fe1c85c5

folly/IndexedMemPool.h

@@ -506,6 +506,9 @@ struct IndexedMemPool : boost::noncopyable {
   void markAllocated(Slot& slot) {
     slot.localNext.store(uint32_t(-1), std::memory_order_release);
   }
+
+ public:
+  static constexpr std::size_t kSlotSize = sizeof(Slot);
 };
 
 namespace detail {