Refactor libs' BN and Point away from monkey patches

[kajoseph]

The monkey patches and prototype overrides in the libs' crypto/bn.js and crypto/point.js cause issues when trying to use the underlying libraries elsewhere in projects.

These need to be refactored to at least use ES2015+ class-based inheritance as opposed to prototype overrides. Ideally, we could get rid of the dependence on BN.js and elliptic's Point altogether.