Newtype for serde

[piegamesde]

The serde support of this crate is just plain awesome. I use #[serde(with = "hex::serde")] all the time. However, I just ran into the situation where I want a Vec<Vec<u8>> to be encoded as a list of hex strings. This is a lot less straightforward to implement, sadly.

To make this more ergonomic, I suggest adding a serialization wrapper newtype that I may use instead of serde_with. I could then write something like Vec<HexBytes> or Vec<Hex<Vec<u8>>> and it would just work.

[piegamesde]

Prior art:
https://docs.rs/serde_bytes/0.11.5/serde_bytes/
serde-rs/bytes#14
jonasbb/serde_with#277
https://docs.rs/serde_with/1.10.0/serde_with/hex/struct.Hex.html