mirage-crypto-rng.unix: provide new generators: Urandom and Getentropy (#250) * Add /dev/urandom and getentropy RNG generators Provide guidance to use these by default, document that Fortuna is not thread-safe. As suggested in #249 * require 4.14 (uses in_channel) * bench/speed: use Urandom and Getentropy generators * mirage-crypto-rng-unix: more documentation * test_entropy: disable on arm64 * Mirage_crypto_rng.generate_into: check off and len being >= 0 * Mirage_crypto_rng.generate_into: adjust docstring * Mirage_crypto_rng.Generator.generate_into: emit unsafe warning Co-authored-by: Reynir Björnsson <reynir@reynir.dk> Co-authored-by: Török Edwin <edwintorok@users.noreply.github.com> Reviewed-by: Calascibetta Romain <romain.calascibetta@gmail.com>