Auto merge of #29580 - alexbool:smart-pointer-conversion, r=alexcrichton

Sometimes when writing generic code you want to abstract over
owning/pointer type so that calling code isn't restricted by one
concrete owning/pointer type. This commit makes possible such code:
```rust
fn i_will_work_with_arc<T: Into<Arc<MyTy>>>(t: T) {
    let the_arc = t.into();
    // Do something
}

i_will_work_with_arc(MyTy::new());

i_will_work_with_arc(Box::new(MyTy::new()));

let arc_that_i_already_have = Arc::new(MyTy::new());
i_will_work_with_arc(arc_that_i_already_have);
```

Please note that this patch doesn't work with DSTs.
Also to mention, I made those impls stable, and I don't know whether they should be actually stable from the beginning. Please tell me if this should be feature-gated.

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