Add ZBox<T> to replace owned variants

[davidcole1340]

Currently types which cannot be owned directly are owned through wrapper types, such as ZendString -> ZendStr, OwnedHashTable -> HashTable, ClassObject -> ZendObject, since they are allocated using the Zend memory manager and cannot be stored in a box.

This PR introduces a new type ZBox<T> and a new trait ZBoxable. A type allocated using the Zend memory manager will implement ZBoxable, and can then be stored inside a ZBox<T>.

There are no 'safe' constructor methods on ZBox. These are to be implemented on the boxed types. For example, ZendStr has a function from_c_str which returns a ZBox<ZendStr>.

[davidcole1340]

At the moment I've only implemented ZBoxable for ZendStr but will do after a quick review. Can be implemented on the following types:

• HashTable
• ZendObject
• ZendClassObject