Implemented Into<[u64;2]> for Xorshift128Plus

[JFT70]

Hi,
Since the internals fields of Xorshift128Plus can not be read and thus restored using a new([u64;2]) I just added support for Serde.

I am working on a seed based procedural generation tool and the ability to store the generator state and reload it is required.

Thank you for your consideration.

[IvanUkhov]

Hello, thank you for the PR. This crate is meant to be minimalistic. I would not want to add that dependency. However, I would be open to opening the internal state if there is interest. If you are up for this, please go ahead and add Into<[u64; 2]>.

[JFT70]

Thank your for your response. I am not used to contributing, I amended my commit. I hope this is the "right way" if not let me know!

[IvanUkhov]

Thank you!