tweak the compiler to assign new spans to use directives

[ibabushkin]

This would help us in some cases to output more helpful spans. However, this depends on #5.

[eddyb]

Note: the compiler has Spans for use items within the same crate, but the export map (which is what gets serialized, not the use items) loses the export chain completely.

You'd only be interested in the use that brought the item into the module, not the rest of the chain, which means you only need to add an extra Span to each re-export, I believe.

[ibabushkin]

The underlying issue is solved now, so I'm closing this.