If rand::OsRng fails, switch to EntropyRng (potential sidedoor)

[samyk]

https://github.com/veorq/oee/blob/6bb7cdf11971e228056fe9108873fdf2aa50812d/oee.rs#L36

Potential sidedoor revealed.

"You may prefer to use EntropyRng instead of OsRng. Is is unlikely, but not entirely theoretical, for OsRng to fail. In such cases EntropyRng falls back on a good alternative entropy source." https://docs.rs/rand/0.5.0-pre.0/rand/os/index.html