Bump num-bigint from 0.2.6 to 0.4.0 (#46)

Bump num-bigint from 0.2.6 to 0.4.0 (#46)

Bumps [num-bigint](https://github.com/rust-num/num-bigint) from 0.2.6 to 0.4.0.
- [Release notes](https://github.com/rust-num/num-bigint/releases)
- [Changelog](https://github.com/rust-num/num-bigint/blob/master/RELEASES.md)
- [Commits](https://github.com/rust-num/num-bigint/compare/num-bigint-0.2.6...num-bigint-0.4.0)

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