Make message on make-package metadata

[rm-hull]

Currently big-bang.package is used to package up a world-state and message as a map. Then there is some mechanics inside big-bang.core, to determine if there is a message in the payload or not.

Rather than having this, we looked at using with-meta previous (see pull request #8), but this wont work for simple world-states that don't implement IWithMeta.

Explore using defrecord instead. I think the only thing that would need to change is the bodies of functions in big-bang.package. The function signatures should stay the same.

There are some existing CLJS tests around this.

Reason for doing this: at some point it would be good to be able to blend other control metadata - such as being able to rewind & fast forward, and this is possibly a fairly transparent way of doing that.