Unable to decode serialized TxOut

There is an issue in serialization of TxOut, we're unable to decode it after serialization.
The problem lyes in the `simplicityhl::elements::encode::serialize`.
For unblinding it misses Rangeproof inside. For some reason it isn't serialized inside.
`TxOut` contains `TxOutSecrets` inside https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/transaction.rs#L703, but `TxOutWitness` has its own serialization https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/transaction.rs#L653 while `TxOut` just truncates it https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/transaction.rs#L719.


Here is a quick code snippet to test it:
```rust
use std::time::Duration;
use simplicityhl::elements::{ Transaction, TxOut,};
use simplicityhl::elements::bitcoin::secp256k1;
use simplicityhl::elements::encode;

pub const PUBLIC_SECRET_BLINDER_KEY: [u8; 32] = [1; 32];
let secret_key = SecretKey::from_slice(&PUBLIC_SECRET_BLINDER_KEY).unwrap();

let tx_id = "8e12b0c994293d15b7f43abaeb6c119ce3612ff00246b7ae9079dca90a35031f";
let url = format!("https://blockstream.info/liquidtestnet/api/tx/{}/hex", tx_id);
let client = reqwest::blocking::Client::builder()
.timeout(Duration::from_secs(10))
.build()
.unwrap();
let tx_hex = client
.get(&url)
.send()
.unwrap()
.error_for_status()
.unwrap()
.text()
.unwrap();

let transaction: Transaction = encode::deserialize(&hex::decode(&tx_hex).unwrap()).unwrap();
let tx_out_from_reqwest = transaction.output[0].clone();
assert!(tx_out_from_reqwest.unblind(secp256k1::SECP256K1, secret_key).is_ok());

let tx_out_from_request_serialized = encode::serialize(&tx_out_from_reqwest);
let tx_out_from_request_deserialized: TxOut = encode::deserialize(&tx_out_from_request_serialized).unwrap();

assert!(tx_out_from_request_deserialized.unblind(secp256k1::SECP256K1, secret_key).is_ok());
```