servo: Merge #18839 - Remove some usage of unstable features (from servo:stabler); r=nox

`Box::new(x)` seems to compile to exactly the same assembly as `box x` in release mode:
https://play.rust-lang.org/?gist=55ec5dd02c22fa799375d6e2e1a6e970&version=nightly

Source-Repo: https://github.com/servo/servo
Source-Revision: fc4de5adf5bff7fabe9f32b960c405f249b5d035

servo/components/script_layout_interface/lib.rs

@@ -7,7 +7,6 @@
 //! to depend on script.
 
 #![deny(unsafe_code)]
-#![feature(box_syntax)]
 #![feature(nonzero)]
 
 extern crate app_units;