Bump bn.js from 5.1.3 to 5.2.0

Bumps [bn.js](https://github.com/indutny/bn.js) from 5.1.3 to 5.2.0.
- [Release notes](https://github.com/indutny/bn.js/releases)
- [Changelog](https://github.com/indutny/bn.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/indutny/bn.js/commits)

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