Adding obslog output options to show precursors instead of/in addition to successors

One of the main bits of feedback we get about obslog is that it's hard to read. Drilling down on that, we found that users expect the opposite logical order - when they look at a node, they want to see how it came to exist, rather than seeing how a node became obsolete. This adds that information to both structured and plain outputs, behind flags.

(recommend viewing the diff for each commit separately)