Update reexports in module vulkano::instance

[DaseinPhaos]

This PR reexports vulkano::instance::layers::LayersListError to make it public-visible.

This is necessary because a former reexport pub use self::layers::layers_list introduces Result<LayersIterator, LayersListError> into the public interface. This in essence actually introduces a private type into the public interface.
Most of the times there would be no problem using values of this smuggled-out type. pub-implemented methods and trait methods can be invoked by outer code just as if the type is actually public visible. However, when the path to this type needs to be explicitly specified, like in the case when a consumer wants to chain this error type with error-chain, the compiler would complain that the type is not in scope. AFAIK the only way to to solve this is via another reexport, making this type really public-visible.

---

A quick example explaining what's going on there:

mod visible {
    pub use self::invisible::bomb_here;

    mod invisible {
        #[derive(Copy, Clone)]
        pub struct Oops { inner: i32 }

        impl Oops {
            pub fn inner(&self) -> i32 { self.inner }
        }

        pub fn bomb_here() -> Oops { Oops { inner : 1 } }
    }
}

fn main() {
    // This is OK:
    let oops = visible::bomb_here(); 

    // `Copy` is OK:
    let ooops = oops;

    // Method invocation is OK:
    let invisible = oops.inner(); 

    // BOOM! This won't compile. Explicitly specifying
    // the type name blows the compiler up:
    let boom: visible::invisible::Oops = oops;
                                     
}

[tomaka]

Thanks