Improve KyberJS performance by switching to BigInt

[Gilthoniel]

There is a native BigInt API in stage 3 (over 4) that is currently available on Chrome and NodeJS. For the moment, it's always work in progress but we may keep that around for the future.

Top is with BigInt and bottom is with bn.js

PS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt