Added OptimResult

[to266]

Implements https://github.com/to266/optimize/issues/9

Also ran cargo fmt on the crate... Will attempt to add this as a requirement for every PR

@sebasv if you could have a look if you agree with the implementation?

• [ ] ready for review

[sebasv]

Regarding the fmt, you can add - cargo fmt --all -- --check to Travis.

[to266]

Just for keeping track of thoughts:

• This PR seems to have started leading me towards stateful optimizers that can be paused/resumed in the middle. While this of course sounds excellent, the changes required to adapt master (and also #12 ) are quite substantial.
• Also, not sure if it's the best implementing the state in a OOP way. I'd like to do this in the data-oriented fashion, but this requires more thinking, since I don't have the experience. Also I'm not sure it's worth it and might result in a better interface / faster / more robust implementation.