Do not use the second MemoryManager in Serializer

* Now that we handle MemoryManager::getMoreMemory(), it is no more needed
  (memory wise) to use the other MM and it actually might be unwise to do so
  (it could allocate an extra heap which might not be needed if there is no GC).
* Also, keeping this would make any sharing of the secondMM more complicated.
* The Serializer does not depend anymore on such low-level memory details.
Introduce dynamic heap management

* heapSize doubles every time we are running out of space after a GC
  or allocated a too big chunk (>= 5%) outside of the _baseBlock.
* allocations when the heap is too filled are done with regular malloc,
  and promptly freed after as we request a GC and grow the size
  to fit the extra alloc (so it should not happen anymore for that block).
* improves the memory footprint and does not increase execution time.
* _maxMemory is kept as an upper bound of memory usage (not yet implemented).