message: Remove five unused features
  Summary: Trying to figure out which features we actually need.
diff: // Features we use
  #![allow(stable_features)]
  #![feature(alloc_layout_extra)]
  -#![feature(backtrace)]
  #![feature(bench_black_box)]
  #![feature(box_patterns)]
  #![feature(box_syntax)]
  -#![feature(cell_filter_map)]
  #![feature(const_fn_fn_ptr_basics)]
  #![feature(const_fn_trait_bound)]
  -#![feature(const_intrinsic_copy)]
  #![feature(const_mut_refs)]
  #![feature(const_ptr_offset)]
  #![feature(const_refs_to_cell)]
  #![feature(core_intrinsics)]
  #![feature(generic_associated_types)]
  #![feature(hash_set_entry)]
  -#![feature(iter_advance_by)]
  #![feature(maybe_uninit_slice)]
  #![feature(maybe_uninit_write_slice)]
  #![feature(ptr_metadata)]
  #![feature(thread_local)]
  -#![feature(try_blocks)]
  #![feature(array_from_fn)]
  #![feature(io_error_more)]
  #![feature(result_option_inspect)]
  