Problem: our types were too loose

[taotetek]

Solution: rework our socket types. This PR:

• Adds a base ZeroMQSocket interface, along with a Client interface for sockets that act as clients (e.g. connect to things) and a Server interface for sockets that act as servers (e.g., bind).
• Adds a ConnectClient function that accepts Client interfaces + an endpoint and attempts to connect the Client to the endpoint
• Adds a BindServer function that accepts Bind interfaces and binds them to an endpoint
• Adds a ClientSocket type that is a ZMQ_CLIENT socket implementing the Client interface
• Adds a ServerSocket type that is a ZMQ_SERVER socket implementing the Server interface

There is still a lot of work to be done to support multiple connections between clients and servers but this lays the groundwork for a more sane type system to build upon.