Stop using JSBI in favour of native BigInt

[troggy]

Bounty

BigInt is supported since Node 10.4+ and is supported by all major browsers (Chrome, Firefox, Edge), so no reason to use JSBI polyfill anymore.

⚠️ Node 8 won't be supported after the change.

Scope

• remove JSBI and jsbi-utils. Replace all the related code with native BigInt arithmetic

Deliverables

• updated code
• updated tests

Gain for the project

cleaner code, less dependencies, less tech debt

Expert

Publicly visible delivery

Roles

bounty gardener: @troggy / 20 DAI bounty worker: @troggy / share bounty reviewer: @roleengineer / share

Funded by

Dev Circle

[troggy]

appears to be bigger issue (see linked leap-core PR). Requesting upsize