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/python/servo/devenv_commands.py

@@ -32,7 +32,7 @@ class MachCommands(CommandBase):
             self.set_use_stable_rust()
             crate_dir = path.join('ports', 'geckolib')
         else:
-            crate_dir = path.join('components', 'servo')
+            crate_dir = path.join('ports', 'servo')
 
         self.ensure_bootstrapped()
         self.ensure_clobbered()