Confusing error message when using non-const `tx_field_id` in `__gtf`

E.g:

```
let x = 0xA;
let _ = __gtf::<raw_ptr>(0, x);
                            ^ Could not evaluate initializer to a const declaration.
```

We should provide a clear error message explaining that `tx_field_id` must be a `u64` constant.
