replace malloc/calloc/free with snapshot_{malloc/calloc/free}

replace malloc/calloc/free with snapshot_{malloc/calloc/free}

We need to explicitly declare when we are requesting snapshotting
memory, and we need to avoid using malloc, etc., directly. For now, this
has no functional change, as the 'snapshot_*' functions just call the
stdlib functions, but soon we may switch allocators to use a private
heap.