Use Reproducible RNG

[ociaw]

Currently we use System.Random. However, while it does take a seed, Random isn't actually guaranteed to return the same numbers across different versions of the runtime. In addition, it has flaws that mean the numbers it generates aren't as high quality as intended. This makes a strong case for moving to a high quality, reproducible RNG instead.

[ociaw]

RandN will be a good candidate for a library for reproducible RNGs when it's released.

[ociaw]

9b0e8fc switched to Pcg32 from RandN, but level and AI seeding will probably be reworked, so it's not stable yet.