Multiplying only works one direction

Multiply only works when called directly or when inverted (`len * 3.0`).

``` rust
#[deriving(Copy, Show)]
pub struct Len(f64);

impl Mul<Len, Len> for f64 {
    fn mul(self, r: Len) -> Len {
        let Len(right) = r;

        Len(self * right)
    }
}

fn main() {
    let len = Len(12.0);

    // works
    println!("{}", 3.0.mul(len));
    // doesn't
    println!("{}", 3.0 * len);
}
```
