Replace ownership wrapper types with `ZBox<T>`

[davidcole1340]

Types such as ZendStr have an owned variant ZendString. This can be generically replaced with a type ZBox<T>, where T implements a new trait ZBoxable, which is implemented on Zend heap-allocated types, such as ZendStr, ZendObject and HashTable:

pub struct ZBox<T: ZBoxable>(NonNull<T>);

pub unsafe trait ZBoxable {
    fn free(&mut self) {
        // by default, just free the memory using the ZMM
        // downstream types like strings and objects can override with their respective release functions
        unsafe { efree(self) };
    }
}

ZBox will dereference to T, and downstream modules such as string can implement on ZBox<ZendStr>, such as impl TryFrom<String> for ZBox<ZendStr>.