rlp: keep only one lib

[yperbasis]

Current state:

• We have low-level functions in erigon-lib/rlp and rlp libs (historically). Target is to use only rlp package.
• We have tx and txpool/tx types (2-nd is subset of 1st).

ToDo:

• [x] low-level functions in rlp. Added: ReadBytes, MoreDataInList. By https://github.com/erigontech/erigon/pull/16036 and https://github.com/erigontech/erigon/pull/16067
• [x] unused new RLP package erigon-lib/rlp2. removed.
• [ ] Move TxPool to use tx type (it will automatically use rlp package)
• [ ] Add rlpgen from geth (see #11116) - but we must change it - to adopt for existing Erigon's codebase. Don't loose our improvements. Rlpgen itself is small - likely easy to change.
• [ ] rlpgen inhereted
• [ ] Get inspiration from Silkworm's RLP code
• [ ] Get inspiration from MarshalJSONTo(*jsontext.Encoder) error https://go.dev/blog/jsonv2-exp
• [x] merge erigon-lib module into Erigon

[Giulio2002]

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

[yperbasis]

TODO: merge rlp/encode.go & rlp/encode2.go.