Rollup merge of #58289 - haraldh:master, r=sfackler

Rollup merge of #58289 - haraldh:master, r=sfackler

impl iter() for dyn Error

Examples:

```rust
let next_error_type_a = err
    .iter()
    .filter_map(Error::downcast_ref::<ErrorTypeA>)
    .next();
```

```rust
let source_root_error = err.iter().last();
```

Credit for the ErrorIter goes to reddit user /u/tdiekmann (Tim Diekmann)
https://www.reddit.com/r/rust/comments/aj3lpg/is_an_iterator_impl_over_errorsource_possible/