Point at fields that make the type recursive

On recursive types of infinite size, point at all the fields that make
the type recursive.

```rust
struct Foo {
    bar: Bar,
}

struct Bar {
    foo: Foo,
}
```

outputs

```
error[E0072]: recursive type `Foo` has infinite size
 --> file.rs:1:1
1 |   struct Foo {
  |  _^ starting here...
2 | |     bar: Bar,
  | |     -------- recursive here
3 | | }
  | |_^ ...ending here: recursive type has infinite size
  |
  = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Foo` representable

error[E0072]: recursive type `Bar` has infinite size
 --> file.rs:5:1
  |
5 |   struct Bar {
  |  _^ starting here...
6 | |     foo: Foo,
  | |     -------- recursive here
7 | | }
  | |_^ ...ending here: recursive type has infinite size
  |
  = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Bar` representable
```

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