Replace BN module with native js BigInt

[ECBSJ]

Why use the BN module over the native js BigInt?

What advantageous did you obtain from using BN and would it be possible to implement the same logic using BigInt and other native js methods?

[lionello]

@ECBSJ This lib is from 2018, before BigInt was supported in all mainstream browsers. It's a good idea to use the native BigInt type now.