[web3.js] Eliminate dependency on bn.js

[steveluscher]

Problem

bn.js represents about 13% of the total size (35K) of the web3.js bundle. It serves a purpose that's now well-handled by JavaScript's native BigInt – an API supported by most major browsers since Sep 2020.

Proposed Solution

• Eliminate dependency on bn.js, replacing it with BigInt

[steveluscher]

This will require that we either replace borsh or ask them to release a new version that only supports bigint: near/borsh-js#60

[github-actions]

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