Small changes from experimental branch

[dhardy]

This is a collection of small changes from my master branch, plus one more (new module for EntropyRng).

• remove next_f32 / 64 — this isn't strictly necessary, but has been discussed and it seems unlikely we'd ever use them (in theory dSFMT is faster, but in practice I suspect not a lot, since random bits from simulation-grade RNGs are cheap and likely outweighed by other operations in most simulations)
• add CryptoRng — also discussed, and requested by a few people even though it neither does nor guarantees anything
• some house-keeping

[pitdicker]

I fully agree with all these changes! Actually wanted to do the same 😄. Feel free to merge from me. You wanted to remove the first commit?

[dhardy]

I don't mind; it's just this and your PR conflict over that bit.

[dhardy]

Okay, I'll merge this. It would be nice to have my first commit merged then your work on top of that I guess.