Additionally:
* Make open2 fallible too
* Use a Result the entire time in open and open2, instead of going from Option to Result
* Specify c_int instead of i32 since apparently the size "may differ on some esoteric systems"; I suspect it won't compile on whatever those are but might as well improve the situation
* Avoid a maybe-possible panic in get_voice_type. Probably can't happen but I'm not 100% certain, so I made it fallible
* Add a missing null check to get_language
* Bump editions to 2018.
* Bump dependencies and crate versions.
* Implement callbacks.
* Return message IDs when speaking, or `None` in case of errors.
* Add callbacks to example.
