Add values_mut() iterator for serde_json::Map

[dtolnay]

This method was added to BTreeMap and HashMap in Rust 1.10.

Will require support from LinkedHashMap. contain-rs/linked-hash-map#82
Mentioning @clarcharr in case you would be interested in sending them a PR.

[clarfonthey]

Wouldn't switching to indexmap be a better choice considering how linked-hash-map isn't actively maintained?

[dtolnay]

Good idea, I would be open to moving to indexmap.