Expose task precedence tracing in the PrecedenceSvc

This MR:

• adds the notion of "cause" to graph traversals (graph visitors are now cause-aware);
• exposes task precedence tracing in the PrecedencSvc;
• cleans up IGraphVisitor, graph visitors' constructors and a helper struct used for assembling the precedence trace graph.

Dependency:

This MR depends on !390 (merged), and branches of it. See fbe3ab3e...47238d77 for the relative differences.