refactor: use bigint directly rather than from the fvm

[lemmih]

Summary of changes

Changes introduced in this pull request:

• BigInt isn't an FVM concept. We should use the type directly from the num-bigint crate.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.