Ensure `ObjectCode` are pickle-able

[leofang]

An ObjectCode instance can encapsulate either PTX, LTO-IR, or CUBIN. All of these can be serialized. This would help us as well as any downstream project implement a simple persistent compiler cache.

[brandon-b-miller]

I can take care of this 👍

[leofang]

btw as discussed in the meeting, the pickle support should be limited to serializing/deserializing the underlying binary blob only; it should not try to touch the driver state, for example. So users unpicking an ObjectCode should still write normal cuda.core code. I don't expect major issues, though, just wanna call this out.