Implement valuable-serde crate

[carllerche]

It should be possible to implement Serde's Serialize trait for valuable::Value. This should enable using Serde's existing serializer ecosystem with valuable.

[carllerche]

@taiki-e is this something you would be interested in taking on? If it is, you can assign the issue to yourself.

[taiki-e]

I've tried it before: https://github.com/tokio-rs/valuable/blob/84c6992b55981181d7b75bb79fcf26428994a019/valuable/examples/serde.rs

If I remember correctly:

• As for implementing Seriarize on Value: I changed some fields such as StructDef::name from &'a str to &'static str, but there were no other major problems.
• As for implementing Valuable on impl Seriarize: It was very difficult and I could not get it to work well.

I will try to update it to the current valuable trait and see if it works.

[taiki-e]

Draft implementation is here: #23