using DIFFICULTY field instead of block header to store the RNG number (EIP 4399)

[SurfingNerd]

EIP 4399 is about storing the Ethereum Random Number in the now useless DIFFICULTY field.
https://eips.ethereum.org/EIPS/eip-4399

Currently we write into the "Difficulty" field the block number, but we could use this space to store the RNG number.
That would save some space for blocks, and we would be more compatible with the ethereum standard.

would require a rewrite of the sync protocol:
#127

[SurfingNerd]

as for the tests done in Alpha 4, we faced Issues in the Sync of the node software.
currently the "Difficulty" field is used to guide the sync -
however, difficulty is in reality always the same than the block number, so we would not loose information with that change.

An improvement of the Sync process could be benificial.