Intertwining was a nice feature, but it slows down hyper significantly,
so it is being removed.
There is some fallout from this, mainly that Incoming has had its type
parameter changed to `<A = HttpAcceptor>` and Handler receiving one
bounded with `A: NetworkAcceptor`.
[breaking-change]
Fixes#112
Expose Server::many for creating a Server that will listen on many (ip, port)
pairs.
Handler still receives a simple Iterator of (Request, Response) pairs.
This is a breaking change since it changes the representation of Listener,
but Handler and Server::http are unchanged in their API.
Fixes#7
