## 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.
---------
Signed-off-by: Arran Schlosberg <519948+ARR4N@users.noreply.github.com>
Co-authored-by: Quentin McGaw <quentin.mcgaw@avalabs.org>
