Prior to this commit, the chunk kept track of the heap ownership
info in the `chunk->actor` field. This worked but was not ideal
as noted in the `ponyint_heap_owner` function that this is a case
of false sharing where the chunk owner needs the chunk but
all other actors only need the chunk owner only. This led to lots
of pointer chasing as the pagemap was used to retrieve the chunk
pointerand then the chunk had to be loaded to retrieve the owning
actor pointer.

This commit removes the `chunk->actor` field further slimming down
both the `small_chunk` and `large_chunk` so both now fit within
32 bytes (on 64 bit architectures). The chunk ownership information
is now kept in the lowest level of the pagemap next to the chunk
information. This removes the previous pointer chasing because
the chunk owning actor pointer is retrieved at the same time as the
chunk pointer without needing to load the chunk itself. This is a
fairly standard tradeoff of memory for performance where we're now
storing more data in the pagemap to minimize pointer chasing. The
expectation is that this will have a net positive impact on
performance but no benchmarks have been run to validate this
assumption.