Please implement Vector<T> From<Point<T>> trait

The `From>` trait is not currently implemented for the `Vector3` struct. Attempting to add this implementation in my own code causes complier error E0117 (Only traits defined in the current crate can be implemented for arbitrary types). It is therefore necessary to implement this `From` trait at source.

As the `Point` struct is based on the `Vector` struct, this should be very easy to implement.