impl Str for CowString

This implementation existed on MaybeOwned, but has been lost in the
transition to Cows. Let's put it back.
auto merge of #19369 : seanmonstar/rust/cow-str, r=alexcrichton

This implementation existed on MaybeOwned, but has been lost in the
transition to Cows. Let's put it back.

@aturon r?