Implement 3SF-mini core for lean consensus

[unnawut]

Describe the feature

It should be possible to reuse most of the code from our rust implementation at https://github.com/ReamLabs/rust-3sf. Take note that some parts like Rc and RefCell might not be needed as they were workaround for simulating networking under a single process.

• Implement block proposal & voting: https://github.com/ethereum/research/blob/master/3sf-mini/p2p.py
• Implement block processing & fork choice: https://github.com/ethereum/research/blob/master/3sf-mini/consensus.py

TODOs

• Move https://github.com/ReamLabs/rust-3sf into ream repo as-is
• Convert Vec to SSZ-compatible VariableList
• Convert HashMap attributes to something SSZ-compatible
• Remove Stakers::num_validators and refer to a fixed validator list in the state instead
• Get rid of Option<Hash> in Block::parent and Block::state_root
• Add proper error handling

Additional context

No response

[unnawut]

Closed by #672