Remove error from to_string / to_vec

[dtolnay]

The to_string and to_vec functions are currently defined to return Result<..., serde_json::Error>. serde_json guarantees that serializing to string will only fail if the type's Serialize impl decides to fail or if the type does not map to legal json, for example because of non-string keys. For syn-serde all of the data structures are designed to be compatible with json and their Serialize impls never decide to fail on their own, so it should be fine to unwrap the errors from serde_json::to_string and remove those errors from the API of syn-serde.

to_writer still needs to return Result because std::io is all fallible.

[taiki-e]

Thanks! Published v0.2.0 which fixes this issue.