Merge the rlp_serialization module in eth_types

[zah]

Having 'rlp_serialization' and 'eth_types' as separate modules is too dangerous. It's quite easy for someone to include only the types module and to invoke the default RLP serialization that doesn't do the right thing for MDigest and Stint values.

Perhaps, we should remove the default serialization altogether and leave it only for tuples, but for now the modified module layout here provides better safety.

[yglukhov]

This could be an option. Another option would be moving rlp_serialization to private dir to keep everything in their relevant files.