style: Use serde to serialize LengthPercentage and StyleRayFunction.

We need to pass these two types into the compositor, so we need a better
way to serialize these rust types. We use serde and bincode to
serialize/deserialize them, and use ByteBuf to pass the &[u8] data
through IPC. We define StyleVecU8 for FFI usage only.

Differential Revision: https://phabricator.services.mozilla.com/D50688

components/style/lib.rs

@@ -78,7 +78,6 @@ extern crate parking_lot;
 extern crate precomputed_hash;
 extern crate rayon;
 extern crate selectors;
-#[cfg(feature = "servo")]
 #[macro_use]
 extern crate serde;
 pub extern crate servo_arc;