Provide accessor for `ObjectCode._handle`

[isVoid]

Currently if we have a lazily loaded ObjectCode, we could get its underlying CUlibrary via _handle attribute. We should provide a public accessor like handle, just like other objects in cuda.core. And it would be good UX if the accessor has a precondition with _lazy_load_module.

[leofang]

@isVoid @ksimpson-work can either of you fix this before Friday? I think this is an overlook from #450/#433 (we did not make ObjectCode a public API so we did not pay attention to ensuring its .handle attribute is exposed). I think like Program/Linker, it probably should have a .backend attribute too?

[ksimpson-work]

I will have a look now

[ksimpson-work]

re backend, I think not because for other objects it refers to different backing modules while in this case it refers to different versions of the same module. That said, I hear the argument in favour and I'm not totally sold either way. Let me know what you think.