Spans of items expanded from macros are funky

[ibabushkin]

Multiple items get assigned the same span. This can be solved in a multitude of ways:

• allow a set of items to be assigned the same span and output all their changes - breaks ordering, tests, and is generally hard to manage correctly.
• serialize macro backtraces and use the correct span at all times, possibly computing the macro's span too. (that's the right way)

[ibabushkin]

This grows ever-more annoying, but at least it seems like the macro spans don't get rendered now.