Choose a PRNG

[LukasKalbertodt]

To always generate the same world, even in future versions of this game, we need to stick to one pseudo random number generator. Currently we're using a xor-shift RNG. But we should discuss and finalize our decision at some point.

[jonas-schievink]

http://www.pcg-random.org/ has very nice properties, and there's a crate. Docs are broken, use this instead: https://crates.fyi/crates/pcg_rand/0.4.1/

[jonas-schievink]

Okay the docs are broken, cargo doc produces something more useful.

[LukasKalbertodt]

Looks nice indeed, although we don't really need many of those properties... speed is important, I guess. So we should test the pcg-rand crate and benchmark, if that implementation holds the speed promises of the website.