Store a pointer to the pool in the free_list

Store a pointer to the pool in the free_list

Since the smallest bin size is big enough to store 2 pointers, the free
list is constructed storing the pointer to the pool the bin belongs to,
so we don't have to find the pool when operating with the free bin.

To make this work, the pools have to be stored outside the DynArray (and
store there only pointers) because when inserting sorted, moved pool
addresses are changed, and the stored pool pointer in the free list break.