Generic vector reads read from the wrong place

Test case:

```
fn get[T](vec[T] v) -> T {
    ret v.(1);
}

fn main() {
    auto v = vec(1, 2, 3, 4, 5);
    log get(v);
}
```

Result:

```
[pwalton@fireferret ~/Source/rust/build] ./a.out
rt: ---
rt: 12ed:main:main:                       rust: 33554432 (0x2000000)
```

`ret v.(0)` works, so looks like we're miscalculating vec element size.
