Spec oracle should be able to return spec traces

[dsheets]

When disagreements occur, it would be nice to have the option for the spec result to include a trace through the spec so that users could easily jump to the issue.

@tomjridge :

Somehow show the branch through fs_spec that lead to the divergence? This would be very nice indeed! But I don't know how to do this. One possibility would be that every "leaf" (eg where a spec ENOTDIR is returned, or where RV_none is returned, or where a special state is signalled) is marked with a unique identifier, but...

The general situation is that the real-world trace diverges from the spec. In general, there is no way to identify where in the spec the behaviour diverged from the real-world. All we know is that the result of the transition is not as observed in the real-world.

For a given result of the spec (eg link abc xyz resulted in ENOTDIR) it would be nice to see how abc and xyz resolved, and the path(s) in the spec that resulted in the ENOTDIR. The debug.native executable can show how the paths resolved. I'm not sure how to produce the path(s) in the spec that lead to ENOTDIR automatically. At the moment, I manually trace through the spec for each failing trace, which is moderately painful to say the least.