message: Remove features array_from_fn, alloc_layout_extra
  Summary: Not used.
diff: // Features we use
  #![allow(stable_features)]
  -#![feature(alloc_layout_extra)]
  #![feature(bench_black_box)]
  #![feature(box_patterns)]
  #![feature(box_syntax)]
  #![feature(maybe_uninit_write_slice)]
  #![feature(ptr_metadata)]
  #![feature(thread_local)]
  -#![feature(array_from_fn)]
  // Plugins
  #![cfg_attr(feature = "gazebo_lint", feature(plugin))]
  #![cfg_attr(feature = "gazebo_lint", allow(deprecated))] // :(
  