HostAddress is in network byte order, but HostAddress6 is in host byte order

So when transferring a v6 address on or off the wire, convert to/from big-endian.  I'm not sure why Network.Socket chose to do it that way, but it's definitely [what it does](http://hackage.haskell.org/package/network-2.5.0.0/docs/Network-Socket.html#t:HostAddress).