Incompatible Value binary serialization format

It appears that the [bincode value fix](https://github.com/ElementsProject/rust-elements/pull/98) resulted in a different serialization format compared to before the [regression](https://github.com/ElementsProject/rust-elements/pull/96) (v0.16).

With current master (fe3e9469618a479de02f8ececf92f9d456fc4c38), the following code:

```rust
let commitment = PedersenCommitment::from_slice(&Vec::from_hex("09b88680778d3d5530785ff12bb62e6ac98c7be5a8b9b0ea392e166bf61ea77ee4").unwrap()).unwrap();
let value = Value::Confidential(commitment);

println!("BE bytes: {}", bincode::serialize(&value).unwrap().to_hex());
println!("LE bytes: {}", bincode::DefaultOptions::default().with_little_endian()
.serialize(&value).unwrap().to_hex());
```

Results in the following serialization:

```
BE bytes: 020000000000000002210000000000000009b88680778d3d5530785ff12bb62e6ac98c7be5a8b9b0ea392e166bf61ea77ee4
LE bytes: 02022109b88680778d3d5530785ff12bb62e6ac98c7be5a8b9b0ea392e166bf61ea77ee4
```

While with v0.16, the following code:

```rust
let value = Value::from_commitment(&Vec::from_hex("09b88680778d3d5530785ff12bb62e6ac98c7be5a8b9b0ea392e166bf61ea77ee4").unwrap()).unwrap();

println!("BE bytes: {}", bincode::serialize(&value).unwrap().to_hex());
println!("LE bytes: {}", bincode::DefaultOptions::default().with_little_endian()
.serialize(&value).unwrap().to_hex());
```

Results in the following serialization:

```
BE bytes: 020000000000000009b88680778d3d5530785ff12bb62e6ac98c7be5a8b9b0ea392e166bf61ea77ee4
LE bytes: 0209b88680778d3d5530785ff12bb62e6ac98c7be5a8b9b0ea392e166bf61ea77ee4
```

使用報告中展示的 Value::Confidential 範例和 bincode 的兩種序列化選項重現此問題。將目前的行為與 v0.16 進行比較，並檢查 PRs 98 和 96 中對 Value 序列化所做的變更。當相容性問題已解決，且產生的位元組已根據報告中的 v0.16 格式完成驗證時，即表示完成。

由索引模型根據 Issue 內容生成。