Features/voltron callbacks

Uses https://github.com/snarez/voltron/pull/49 to get calculon to update when a voltron hooked debugger stops.

Like an idiot I didn't base this work on the back that can watch expressions, making it pretty useless. I'm gunna hold off on fixing that merge conflict until it gets onto master though since I figure that's likely to change.

Also, the redraw never happens on my system, although printf debugging confirms the callback fires, so I suspect that it's because blessings can't work my terminal.
Yeah, that's why I hadn't split it up already :(