Float::exp2() is calling exp()

This succeeds and shouldn't. 

``` rust
let f = 2.0f32;

assert_eq!(f.exp2(), f.exp());
```

In [`f64`](https://github.com/rust-lang/rust/blob/master/src/libstd/num/f64.rs#L186), `f64` is calling `exp`. In [`f32`](https://github.com/rust-lang/rust/blob/master/src/libstd/num/f32.rs#L176), `f32` is also calling `exp`. Might just have to add a 2...
