Implement Mul for Transform

[silverweed]

Since chaining multiple calls to Transform::combine is inconvenient, imho the usability of Transform would benefit from having the Mul trait implemented.

I'm guessing the implementation would go something like this:

impl Mul for Transform {
    type Output = Transform;

    fn mul(mut self, rhs: Transform) -> Transform {
        self.combine(&rhs);
        self
    }
}

(However, being relatively new to Rust I don't know if there are any caveats preventing this implementation from working correctly.)