Doesn't work with Rust 2018 edition `dyn` traits

[thomaseizinger]

If you have a struct that uses the new Rust 2018 dyn trait syntax, the custom derive panics:

pub trait Foo {

}

#[derive(DebugStub)]
pub struct Bar {
    #[debug_stub = "foo"]
    foo: Arc<dyn Foo>
}

[D4nte]

Any chance for this to be looked at? :)