Don't expose internal info

[gaearon]

Looking at #6 again, I understand now that onUpdate and shouldUpdate "leak" internal instances to the user code. Can we please avoid this? Let's change the code to take the limited amount of information you need (e.g. an object containing the fields you're interested in). We can add more fields but I don't want to make internal structure exposed.

@bfiss Could you please change this?

[gaearon]

That would also make it work identically with both Stack and Fiber. Current code will pass different things depending on the engine.