Much like it makes sense to enumerate all the families,
it makes sense to enumerate stuff within families.
Alternatively, we could have a global instance index,
but i'm not sure why that would be better.
This will be useful when the benchmarks are run not in order,
for the tools to sort the results properly.
It may be useful for those wishing to further post-process JSON results,
but it is mainly geared towards better support for run interleaving,
where results from the same family may not be close-by in the JSON.
While we won't be able to do much about that for outputs,
the tools can and perhaps should reorder the results to that
at least in their output they are in proper order, not run order.
Note that this only counts the families that were filtered-in,
so if e.g. there were three families, and we filtered-out
the second one, the two families (which were first and third)
will have family indexes 0 and 1.
