vec::view()'s signature is unsafe

```
pure fn view<T>(v: &[T], start: uint, end: uint) -> &a.[T]
```

should be either

```
pure fn view<T>(v: &[T], start: uint, end: uint) -> &[T]
```

or

```
pure fn view<T>(v: &a.[T], start: uint, end: uint) -> &a.[T]
```

(edit: thanks, niko)
