Implement permissive BFT

[mdimjasevic]

Implement permissive BFT

[edsko]

Note that this depends on two improvements on the ledger side:

• Make the Block type polymorphic #304
• Split block header processing into protocol and payload components #305

[edsko]

Ok, so, change of plan. New plan is as follows:

• Ensure a clearer separation between "consensus stuff" and "ledger stuff" in the ledger spec (input-output-hk/cardano-ledger-byron#305)
• Generalize the "consensus stuff" part so that it does not require a specific type of Block but is more polymorphic (input-output-hk/cardano-ledger-byron#304). Note that there is no need to generalize the "ledger part", that can stay using a monomorphic Block type.

We would know we achieved our goal if we can run the Permissive BFT using the spec in our tests, using our SimpleBlock as the block type and our mock ledger (Ouroboros.Consensus.Ledger.Mock).

This means that modifying the tests to use the "real" ledger can then properly be a separate task altogether (#243), and once done, we should then easily be able to run the real ledger with permissive BFT (#244), as well as other combinations.

[edsko]

Note on time estimate: allowing a fair amount of time here, since this is the first integration point and is therefore bound to require some massaging on both ends.

[mdimjasevic]

Now that there is a protocol implementation for the permissive BFT, per @edsko the next step should be to get to a point where we can run the test-consensus test suite with PermBFT as the consensus algorithm.

[edsko]

Closing this as WONTFIX; we will only have the "real" implementation (#274).