message: core: fix `spin_loop_hint` deprecation
  Allow the deprecated API for now, until the next MSRV bump.
diff: use core::cell::UnsafeCell;
  use core::fmt;
  -use core::sync::atomic::{spin_loop_hint as cpu_relax, AtomicUsize, Ordering};
  +use core::sync::atomic::{AtomicUsize, Ordering};
  +// TODO(eliza): replace with `core::hint::spin_loop` once our MSRV supports it.
  +#[allow(deprecated)]
  +use core::sync::atomic::spin_loop_hint as cpu_relax;
  /// A synchronization primitive which can be used to run a one-time global
  /// initialization. Unlike its std equivalent, this is generalized so that the
  