[EIP 2384]: Implement difficulty bomb delay

[evertonfraga]

EIP 2384 is still in draft mode, but should escalate to a final state quickly, as the target date is mid-January.

Requirements:

• A fork needs a name, to implement in -common. Will use EIP-2384 for now
• Block number from EIP
• Difficulty tests from ethereum/tests
• Copy tests to this repo and add relevant metadata (source, date, commit and hash)

[holgerd77]

For reference here the difficulty bomb implementation for Constantinople: #54

@evertonfraga I am not following the discussion closely, is this rather planned for the Berlin HF, will this be an own HF or is this not decided yet (optimally directly link to the discussion)?