Serialize number to no more than necessary precision I'm not sure whether it can be simplified... and I probably need some more idea for how should I test it... But things look fine so far. This should probably goes to an independent crate at some point. It should fix servo/servo#17205. <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-cssparser/173) <!-- Reviewable:end -->