RNG: squashed commits from @infinity0

* Estimation of seed entropy decreased to 4 bits per byte.
* System RNG assumption relaxed down to it is secure at generating values (though it's assumed insecure at seeding).
* Put some claritications about brute-force attack cost on `weak_seed` routine.

See https://github.com/vibornoff/asmcrypto.js/issues/27
BigNumber: modulo inverse fix, RSA keygen fix & comments
fix mixing in of the seed in ISAAC PRNG, this should have been part of ab5a6a6d