Supertrait shown in docs for core::num::Zero but not std::num::Zero.

I imagine this has something to do with re-exporting:

http://static.rust-lang.org/doc/master/core/num/trait.Zero.html

```
pub trait Zero: Add<Self, Self> {
    fn zero() -> Self;
    fn is_zero(&self) -> bool;
}
```

http://static.rust-lang.org/doc/master/std/num/trait.Zero.html

```
pub trait Zero {
    fn zero() -> Self;
    fn is_zero(&self) -> bool;
}
```
