Format events in their span contexts

[hawkw]

Currently, when the console prints out events, it does not show the span context in which they occurred. As far as I can tell, there isn't really any way to show span data currently.

I think that spans should probably be formatted with fields from their parent spans (and possibly the names of those spans as well), similarly to how tracing-fmt prints out the shaving_yaks and shave spans in this example:

(of course, the particulars of how spans are actually formatted need not be identical)

The ability to show/hide these span contexts could also be added, in the future.