blstrs Zeroize feature

[gilcu3]

Background

The upstream library blstrs crate does not support Zeroize for sensitive values. For this reason in #237 we had to introduce some unsafe code. We should open a PR upstream to add this feature.

User Story

Avoiding having unsafe code in this PR is better.

Acceptance Criteria

A PR upstream is opened and merged that adds support for Zeroize in the Scalar type

Resources & Additional Notes

No response

[gilcu3]

Opened PR upstream filecoin-project/blstrs#66