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_traits/lib.rs

@@ -22,7 +22,6 @@ extern crate malloc_size_of;
 #[macro_use]
 extern crate malloc_size_of_derive;
 extern crate selectors;
-#[cfg(feature = "servo")]
 #[macro_use]
 extern crate serde;
 extern crate servo_arc;