Can't clone `&T` when `T` is unsized

[japaric]

STR

Found while attempting #19036

struct Str([u8]);

#[deriving(Clone)]
struct CharSplits<'a, Sep> {
    string: &'a Str,
    //~^ error: the trait `core::kinds::Sized` is not implemented for the type `Str`
    sep: Sep,
    allow_trailing_empty: bool,
    only_ascii: bool,
    finished: bool,
}

fn clone(s: &Str) -> &Str {
    Clone::clone(&s)
    //~^ error: the trait `core::kinds::Sized` is not implemented for the type `Str`
}

fn main() {}

We need to DSTify the impl<'a, T> Clone for &'a T

cc @aturon
cc #16918

[aturon]

@japaric Note, this is one of the cases that came up with git grep "impl.*for &" -- would be good to finish going through those impls.