impl Filecoin.MinerCreateBlock

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• implemented Filecoin.MinerCreateBlock,
• changed wallet import so that it allows specifying the key via stdin non-interactively,
• both Lotus and Forest now share the same wallet key,
• first write tests - as a follow-up I'll uncomment ones that were blocked by this.

Reference issue to close (if applicable)

Closes #4202

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.

[aatifsyed]

I'd love to see the Miner methods in state.rs moved while you're at it :)

[LesnyRumcajs]

I'd love to see the Miner methods in state.rs moved while you're at it :)

Sure, I'll do it in a follow-up tomorrow.