message: Fix infinite recursion in `Debug` impl for `FieldSet`
  I had never actually tried to print a `FieldSet` before, but it turns
  out that doing so will recur infinitely and cause a stack overflow.
  Fixed that.
diff: @@ -194,7 +194,10 @@ impl<'a> IntoIterator for &'a FieldSet {
  impl fmt::Debug for FieldSet {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  - f.debug_set().entries(self).finish()
  + f.debug_struct("FieldSet")
  + .field("names", &self.names)
  + .field("callsite", &self.callsite)
  + .finish()
  }
  }
  