New and improved library and macro API

## Library API

```rust
pub trait RlpEncodable: RlpLength {
fn rlp_encode(&self, encoder: &mut Encoder);

fn rlp_encode_raw(&self, encoder: &mut Encoder);

fn rlp_len(&self) -> usize {
let raw_len = self.rlp_len_raw();
length_of_length(raw_len) + raw_len
}

fn rlp_len_raw(&self) -> usize; // Important: no default!!
}

// Might not be needed.
// pub trait DynRlpEncodable: RlpLength { /* ... */ }
// pub trait RlpLength { /* rlp_len, rlp_len_raw */ }
// impl DynRlpEncodable for T where T: RlpEncodable { /* ... */ }

pub trait RlpDecodable<'de> {
fn rlp_decode(decoder: &mut Decoder<'de>) -> Result;

fn rlp_decode_raw(decoder: &mut Decoder<'de>) -> Result {
Self::rlp_decode(decoder)
}
}

pub struct Encoder {
out: T,
}

// `struct Rlp` is replaced with `Decoder`.
pub struct Decoder<'de> {
buf: &'de [u8],
}

impl<'de> Decoder<'de> {
// Methods moved from `Header` ...

// Methods for creating errors that contain source information like byte position
}

pub struct Error {
bytepos: usize,
kind: ErrorKind,
}

pub enum ErrorKind {
// Previous `enum Error` ...
}
```

Other, not pictured:
- [ ] Add trait implementations for tuples, with the same behavior as structs with derived impls
- This with the improved macro attributes should remove the need of `dyn Encodable`, enabling use of a generic `Encoder`.

Improvements:
- trait and method names prefixed with `rlp` to avoid conflicts on common types
- separate methods for "raw" encoding/decoding, meaning without header, to allow `#[rlp(flatten)]`, see below

## Derive macros

New attributes:
- [ ] (field) `#[rlp(flatten)]`: encode and decode using `raw`
- [ ] (container) `#[rlp(tagged)]`: impls for enums
- `tagged`: defaults to the variant's discriminant value, or the variant can have an `#[rlp(tag = )]` attribute.
- Example: [`P2PMessage`](https://github.com/paradigmxyz/reth/blob/3e8d5c69cf67906909d41fa5b2737bd9667e46e5/crates/net/eth-wire/src/p2pstream.rs#L649)
- not specifying this fails, to leave room for future enum representations
- [ ] (container) `#[rlp(transparent)]`: replaces the separate `*Wrapper` macros
- also account for `#[rlp(skip)]` in field number calculation