Major enhancement: addition of debugging UI elements

[jon-axon]

This is a placeholder issue and should be replaced by/broken down into a series of smaller enhancements in due course.

Although currently unused, the Chipolata library crate already exposes through the StateSnapshot::ExtendedSnapshot enum variant all the information that would be required for the UI to provide full de-bugging information to the user. So, combined with the existing capability to pause and restart execution and run cycle-by-cycle when required, there should already be sufficient support within the library to facilitate a fully-fledged debugging UI (perhaps without the ability for the user to manipulate state).

Thought should be given to how to best represent this from a UI "real estate" perspective, what the overall scope should be, and whether to incrementally add these capabilities or to fold into a single "debugging update" release.