PCG extension mechanism

[pitdicker]

About a month ago I started implementing the PCG extension mechanism in a repository where I compare small RNG's. But when I realized how complex it was it did not seem like a good fit.

I you are still interested in working on the extension mechanism, it may be a good starting point:
https://github.com/pitdicker/small-rngs/blob/master/src/pcg_ext.rs
And an overview of how I think it is supposed to work: dhardy/rand#52 (comment)

[pitdicker]

And the C++ code this translates (the necessary functions extracted from PCG): gist

[robojeb]

@pitdicker Sorry for being a bad maintainer, I have implemented extended generators now.

They are based on some of the code from C++, though because we don't have const-generics we are limited to just a small set of sizes.