std: Rename AsRef{Reader,Writer} to ByRef{Reader,Writer}
auto merge of #18891 : erickt/rust/deprecate-as-ref, r=alexcrichton

It seems odd that the `AsRefReader`/`AsRefWriter` have the single method `by_ref()`. This creates the new traits `ByRefReader`/`ByRefWriter` and deprecates the old traits.