message: Add some missing #![feature]s. Fixes "make doc".
diff: * and Eric Kidd's toy OS (https://github.com/emk/toyos-rs).
  */
  -#![feature(abi_x86_interrupt, alloc, allocator_api, asm, attr_literals, const_fn, global_allocator, iterator_step_by, lang_items, linkage, repr_align, specialization)]
  +#![feature(abi_x86_interrupt)]
  +#![feature(alloc)]
  +#![feature(allocator_api)]
  +#![feature(asm)]
  +#![feature(attr_literals)]
  +#![feature(const_fn)]
  +#![feature(const_atomic_bool_new)]
  +#![feature(const_atomic_usize_new)]
  +#![feature(const_unsafe_cell_new)]
  +#![feature(global_allocator)]
  +#![feature(iterator_step_by)]
  +#![feature(lang_items)]
  +#![feature(linkage)]
  +#![feature(repr_align)]
  +#![feature(specialization)]
  #![no_std]
  // EXTERNAL CRATES
  