RNG: `weak_seed` fix

https://github.com/vibornoff/asmcrypto.js/issues/27#issuecomment-45215222

* Phase out `weak_seed` entropy estimation at all since it's generally useless.
* Change `weak_seed` PBKDF2 logic in a way described at https://gist.github.com/vibornoff/adc89b5ef0d63f52f459