Missing Null Pointer Optimization Through Struct Indirection

Given a small wrapper struct around a box:

``` rust
struct Wrapper<T> {
    ptr: Box<T>
}
```

Currently there is no null pointer optimization for a usage like `Result<(), Wrapper<X>>` whereas there is for `Result<(), Box<X>>`.  The result in the first case will be two words whereas in the latter a single.
