Port to rand_core 0.2

[vks]

I'm working on updating pcg_rand to use rand_core 0.2 (which corresponds to Rand 0.5) instead of Rand 0.4. Unfortunately, I ran into a compiler bug (rust-lang/rust#43408). I'm not sure yet how to work around it.

[robojeb]

Hi @vks, I just pushed a new version based on the Rand 0.5.
Does this work for you now?

[vks]

Looks good, thanks for the update!