Seemingly valid bit vector indexing yields parse errors

Code is below. Errors and the problematic code are in the comments

```rust
# my_lib
pub const W = s32:16;
pub const Z = s32:24;
```

```rust
import my_lib;

pub const A = s32:16;
pub const B = s32:24;

fn foo(x: u32) -> () {
// 0013: let a = x[my_lib::W : s32:24];
// ~~~~~~~~~~~~~~~~~^------------------^ TypeInferenceError: uN[32] Value to index is not an array.
// let a = x[my_lib::W : s32:24];

// 0017: let b = x[my_lib::W : my_lib::Z];
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^ ParseError: Old-style cast only permitted for constant arrays/tuples and literal numbers.
// let b = x[my_lib::W : my_lib::Z];

// OK.
let c = x[A:my_lib::Z];

// OK.
let d = x[A:B];

()
}
```