Allow constant struct fields and tuple indexing
Add tests for issue 19244
auto merge of #19266 : aochagavia/rust/const, r=alexcrichton

With this PR, the following code works:

```
#![feature(tuple_indexing)]
struct MyStruct { field1: uint }

const S: MyStruct = MyStruct { field1: 42u };
const T: (uint,) = (42u,);

struct ConstCheck {
    array1: [int, ..S.field1],
    array2: [int, ..T.0],
}
```

Closes https://github.com/rust-lang/rust/issues/19244
Related https://github.com/rust-lang/rust/issues/19265