A slower reflection-based arena that can allocate objects of any type.

This arena uses Vec as a backing store to allocate objects from. For each allocated object, the arena stores a pointer to the type descriptor followed by the object. (Potentially with alignment padding after each element.) When the arena is destroyed, it iterates through all of its chunks, and uses the tydesc information to trace through the objects, calling the destructors on them. One subtle point that needs to be addressed is how to handle failures while running the user provided initializer function. It is important to not run the destructor on uninitialized objects, but how to detect them is somewhat subtle. Since alloc() can be invoked recursively, it is not sufficient to simply exclude the most recent object. To solve this without requiring extra space, we use the low order bit of the tydesc pointer to encode whether the object it describes has been fully initialized.

As an optimization, objects with destructors are stored in different chunks than objects without destructors. This reduces overhead when initializing plain-old-data and means we don't need to waste time running the destructors of POD.