add freelist allocator to allow recycling of chunks with ChunkedBumpAlloc

[michaelsippel]

With the the current memory alloctor , ChunkedBumpAlloc will deallocate a chunk , once all its allocations contained in this chunk are deallocated. In practice this means deallocated hwloc_free.
Whenever new memory is requested , another hwloc_alloc_membind is executed.

A potential FreelistAllocator , plugged in between HwlocAlloc & ChunkedBumpAlloc , should keep track of a (limited) number of free chunks and return them to ChunkedBumpAlloc whenever possible.