[WIP] Improve RLP encode/decode core types

[racytech]

This PR will fix RLP related issues, removing some unused parts and improving some other parts of the code. Benchmarks on Log type encoding/decoding shows that it makes sense to keep using hand-written code.

[racytech]

Moved RLP logic in core types to separate file

[racytech]

Moved often used rlp functions from ./rlp/encode to ./erigon-lib/rlp/encode

[AskAlexSharov]

Benchmarks on Log type encoding/decoding shows that it makes sense to keep using hand-written code. - did you compare it with reflection-based code or with rlpgen-based?

[racytech]

With reflection-based, on Log struct only

[racytech]

let me test it against rlpgen

[racytech]

RLPgen: 492243439 48.56 ns/op 0 B/op 0 allocs/op
ByHand: 312318904 79.10 ns/op 144 B/op 4 allocs/op

We should move towards rlpgen, if no objections, didn't know what that is :)

[racytech]

Silkworm based approach + reusable buffer: 667179529 35.36 ns/op 0 B/op 0 allocs/op