[core] Add transaction listener interface

[gburd]

This is the Rust equivalent of #61 and Datomic's Transaction Report Queue. The idea is to push transaction reports to a queue (or invoke a callback, or ...) to allow the embedding application to react to specific types of transactions. There are lots of ways to achieve this, all with different trade-offs.

If we want to push mozilla/tofino#1575 all the way across the line, we'll need some form of transaction listener to push changes to the WebSocket clients.

This depends on #296.