`rand_core` v0.9 support

[tarcieri]

rand_core v0.9 is out

We will need to branch for a v0.7 release series before this happens. See #752

[baloo]

Fixed in #762

[fjarri]

Would it be possible to publish a pre-release with rand=0.9 so that dependent crates could fix the version?

[tarcieri]

Sure