Change interfaces to structs.

Started with some global search and replace, then cleaned up as
necessary to get things building and passing tests again.

This change also includes a "zmqstructs" rewrite for gozmqfix.  Updating
existing code should be as simple as:

    go install github.com/alecthomas/gozmq/gozmqfix
    gozmqfix PATH...
Merge pull request #70 from jtacoma/struct-to-interface

Publish the hidden zmqContext and zmqSocket structs as Context and Socket, replacing the Context and Socket interfaces.