Use ToSocketAddr to allow the string form of a network address

Using the `ToSocketAddr` trait, we can eliminate the need to import `Ipv4Addr` every time.
Oh, I misread this code here. If the IronListener was created in this thread then sent over it would be fine, but since it is created inside the proc we do send addr.

I'd rather not have the Send bound, so why we just create the IronListener outside the proc then move it into the proc to call .serve.