ISAAC: don't reset when seeding

https://github.com/vibornoff/asmcrypto.js/issues/27
PRNG: tweak the mechanics of Random.getValues
- use crypto.getRandomValues directly when available, instead of using it to seed ISAAC
- add an entropy estimator, and forbid manual seeds with low entropy
- split the weak seeding into a separate function so the logic is clearer
- greatly reduce the documentation of the weak seeding, we don't want users to rely on it

https://github.com/vibornoff/asmcrypto.js/issues/27
ISAAC: refactor ISAAC seeding function to be closer to the original implementation

https://github.com/vibornoff/asmcrypto.js/issues/27
PRNG: set a weak seed unconditionally, to raise the cost of an attack on a weak system RNG
https://github.com/vibornoff/asmcrypto.js/issues/27
PRNG: explicitly allow for the possibility that the system RNG is weak

https://github.com/vibornoff/asmcrypto.js/issues/27
PRNG: Some of @infinity0's changes reverted