Migrate to `Total-Random/pcg-cpp`

[ikrommyd]

From imneme/pcg-cpp#106 and https://www.reddit.com/r/cpp/comments/1pt25rg/maintaining_the_legacy_totalrandom_takes_over/, it looks like there is a new maintained fork of pcg-cpp that lives under https://github.com/Total-Random/pcg-cpp
Should we migrate to that?

[ikrommyd]

This also has imneme/pcg-cpp#99 merged allowing #292 to move forward

[nsmith-]

Seems like a good idea

[ikrommyd]

I hope that just updating the submodule is enough and there are other differences.