Boxes are a bit magic in that they need to use `*` to get an owned value
out of the box. They implement `Deref` but that only returns a
reference. This means an easy way to convert an `Option<Box<T>>` to an
`<Option<T>` is:
```
box_option.map(|b| *b)
```
However, since b36bb0a6 the `map_clone` lint is detecting this as an
attempt to copy the box. Fix by excluding boxes completely from the
deref part of this lint.
Fixes#3274
