[CSPRNG] Provides CSPRNGs

[mratsim]

We need to provide at least system random for seeding.

See also

• Parallel Random Numbers: As Easy as 1, 2, 3
  John K. Salmon, Mark A. Moraes, Ron O. Dror, and David E. Shaw
  https://thesalmons.org/john/random123/papers/random123sc11.pdf
  https://github.com/DEShawResearch/random123

• Fast-key-erasure random-number generators
  Daniel J. Bernstein
  https://blog.cr.yp.to/20170723-random.html
  https://github.com/floodyberry/supercop/blob/a351f2c/crypto_sign/ntrumls439x/ref/fastrandombytes.c

• Randen - fast backtracking-resistant random generator with AES+Feistel+Reverie
  Jan Wassenberg, Robert Obryk, Jyrki Alakuijala, Emmanuel Mogenet
  https://arxiv.org/abs/1810.02227
  https://github.com/google/randen

[mratsim]

The OS PRNG is provided by #257