private key from keypair tool

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• a simple tool to get the private key from the keypair so that it can be used in Lotus and vice-versa. Theoretically, we could make the import/export more user-friendly, but then we'd be in the field of compatibility with Lotus internal structure, which is not guaranteed to be the same across versions.

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.

[samuelarogbonlo]

Nice