Deserializer: Give client ownership of events about yaml-rust

[dtolnay]

I agree, it should hand over the event by value. We would welcome a PR!

[chris-m-h]

When opening this issue I assumed that this project was a push parser.

However, after separating parser and event emitter while implementing PR #75, I realized that it is in reality a pull parser that is encapsulated in a push parser. The latter only validates the state machine model of the former.

Now I am not sure about the underlying reasons, but in my understanding, pull parsers are much more client friendly. I think it would be easy to change the parser to a pull parser. If there is some interest in that I could propose a pull request once I have a bit of time.