Clean up and improve our RLP code

[yperbasis]

Currently our RLP code is a mess. There're the following disparate libraries & approaches:

• rlpgen inhereted from geth (see #11116)
• low-level functions in rlp
• low-level functions in erigon-lib/rlp
• unused new RLP package erigon-lib/rlp2

We should simplify and improve this zoo, perhaps also taking silkworm's RLP code into consideration. If we decide to use low-level hand-written code instead of rlpgen, we should measure performance and do that only in case the performance gain is significant enough to justify the pain of hand-written boilerplate.

[Giulio2002]

another thing I would like to add is to add the ability to implement reusable buffers