Make the serde support behave better The previous implementation could produce illegal JSON when used with serde_json. This is a breaking change because we have to add a Clone bound on the Deserialize impls. <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/euclid/141) <!-- Reviewable:end -->