BlockRng64 doesn't implement CryptoRng

[Pr0methean]

Problem: Although rand has the declaration

impl<R: BlockRngCore + CryptoRng> CryptoRng for BlockRng<R> {}

there's no equivalent for BlockRng64<R>, so a BlockRng64 can't pass any compile-time check for a CryptoRng even when the core does.

[dhardy]

This was already changed in master: #1273

And, yes, that's not released yet; there's a tracking issue for version 0.9 if you're interested.