This patch adds pipenet - virtual network of net.Pipes.

Addresses on pipenet are host:port pairs. A host is xnet.Networker and so
can be worked with similarly to regular TCP network with Dial/Listen/Accept/...

Example:

    net := pipenet.New("")
    h1 := net.Host("abc")
    h2 := net.Host("def")

    l, err := h1.Listen(":10")       // starts listening on address "abc:10"
    go func() {
            csrv, err := l.Accept()  // csrv will have LocalAddr "abc:10"
    }()
    ccli, err := h2.Dial("abc:10")   // ccli will have RemoteAddr "def:10"

Pipenet might be handy for testing interaction of networked applications in 1
process without going to OS networking stack.