Minor cleanups.

This PR updates a few parts to be a little more Go-idiomatic
without changing any significant semantics.

Package comments are attached to the package statement so that they'll
show up in godoc.

GoSend is changed to a simple function, which arguably looks slightly
nicer and uses one less allocation per send. It will also work better
if Message is changed to be an interface type, which might be somewhat
more idiomatic (with one concrete type per message type).

A couple of race conditions cleaned up in graphml.

A few other little tweaks.