refactor(core/types): simplify Body RLP override

[ARR4N]

Why this should be merged

Simplification of types.Body RLP overriding, resulting in reduced code at both the implementation and consumer ends.

How this works

Introduction of rlp.Fields type, to mirror regular RLP encoding of a struct. The RLP override hook now only needs to return the fields of interest, which MAY come from either the Body or the registered extra.

This pattern allows for arbitrary modification of upstream fields via (1) reordering; (2) addition; (3) deletion; and (4) inverting required vs optional status. While less important for Body, this allows for complete support of ava-labs/coreth Header modifications, which make use of 1-3.

How this was tested

Existing backwards-compatibility tests + new unit tests for introduced functionality.

[darioush]

if we are adding something like rlp.Nillable, does it make sense to also add rlp.Optional?
It seems like a better callsite experience than returning 2 []any

[ARR4N]

I thought of this but then got stuck on what the rest of the API looks like (e.g. what does EncodeStructField() accept? Do non-optional fields also have to be wrapped in something?). I'll ponder it a bit more, experiment with the rest of the code, and get back to you.

[ARR4N]

Done. WDYT? It feels a little bit clunky inside the implementation (hence the explanatory comment), but so much clearer here so I completely agree that:

It seems like a better callsite experience than returning 2 []any

When I originally just introduced type Optional []any, so the usage would have been rlp.Optional{a,b,c} (instead of parentheses), it still compiled with just []any. That's why I introduced the OptionalFields type, to force the clearer call sites.

[darioush]

Now that we have gone this far, what do you think about adding rlp.Required as well? So the callsite can look like:

return rlp.Required(b.Transactions, b.Uncles), rlp.Optional(b.Withdrawals)

instead of:

return []any{b.Transactions, b.Uncles}, rlp.Optional(b.Withdrawals)

[ARR4N]

It became unwieldy and ugly so I did what I should've done all along and changed it to:

return &rlp.Fields{
  Required: []any{b.Transactions, b.Uncles},
  Optional: []any{b.Withdrawals},
}

Everything is much cleaner now; call sites and implementation.

[qdm12]

Pretty cool 👍 !