[Feature] Migrate to native BigInt

[mahnunchik]

Hello,

Since you are working on the next major release, it would be very useful to migrate to native BigInt from BigNumber.

It is widely accepted https://caniuse.com/bigint

Many crypto libraries has been migrate to native BigInt in past year.

[mahnunchik]

Even Buffer can be replaced with native UInt8Array

[mahnunchik]

An example of successful migration: https://github.com/XRPLF/xrpl.js/releases/tag/xrpl%403.0.0

[ashisherc]

I will keep these for v3, which is due with the next Cardano HF.