message: Actually remove CURRENT_SPAN from core
  Apparently I forgot to in
  Not sure why the compiler didn't notice that was unused.
diff: //! Spans represent periods of time in the execution of a program.
  use std::{
  borrow::Borrow,
  - cell::RefCell,
  cmp, fmt,
  hash::{Hash, Hasher},
  sync::atomic::{AtomicBool, AtomicUsize, Ordering},
  @@ -13,11 +12,6 @@ use {
  Dispatch, Meta,
  };
  -thread_local! {
  - // TODO: can this be a `Cell`?
  - static CURRENT_SPAN: RefCell<Option<Enter>> = RefCell::new(None);
  -}
  -
  /// A handle representing a span, with the capability to enter the span if it
  /// exists.
  ///
  