Make it possible to see the call stack from the debugger

[bbatsov]

This is a continuation of a short discussion with @arrdem on Slack. Right now you have to throw an exception to see the call stack, while in the debugger. Surely we can do better. :-)

[Malabarba]

Thanks for the reminder. I wrote this on the airport a couple days ago. I'll file a PR when I things settle down a bit here.