vectors don't handle zero-size elements correctly

Any usage of `offset` from the `ptr` module is likely wrong, because it uses `size_of` instead of `nonzero_size_of`. The iterators and `slice` implementations are two examples.

``` rust
struct Foo;
let xs = ~[Foo, Foo, Foo];
println(fmt!("%?", xs.slice(0, 2).to_owned())) // prints `~[]`
```
