EVM Node: Fix RLP encoding for transaction receipts

In the kernel, the index of the transaction receipt is a u32, using the default RLP encoding for the primitive type (see etherlink/kernel_evm/ethereum/src/transaction.rs).

let index: u32 = decode_field(&next(&mut it)?, "index")?;

A peek at the rlp crate to see how the decoder is defined teaches us that u32 are encoded as big-endian numbers (without the leading 0s).

macro_rules! impl_encodable_for_u {
  ($name: ident) => {
    impl Encodable for $name {
      fn rlp_append(&self, s: &mut RlpStream) {
        let leading_empty_bytes = self.leading_zeros() as usize / 8;
        let buffer = self.to_be_bytes();
        s.encoder().encode_value(&buffer[leading_empty_bytes..]);
      }
    }
  };
}

On the node side, we can find the encoding of the transaction receipts in etherlink/bin_node/lib_dev/encodings/transaction.ml, and

let index = decode_number_le index in

And here it is. That is why the result for eth_getTransactionReceipt is inconsistent for blocks with more than 256 transactions.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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• Provide automatic testing (see the testing guide).
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