WIP: Allow const struct fields and tuple indexing

With this PR, the following code should work:

```
#![feature(tuple_indexing)]
struct MyStruct { field1: uint }

const S: MyStruct { field1: 42u };
const T: (uint,) = (42u,);

struct ConstCheck {
    broken_1: [int, ..S.field1],
    broken_2: [int, ..T.0],
}
```

Closes https://github.com/rust-lang/rust/issues/19244