llvm dev - Dec 2020 - Trying to use unordered_map

Indeed, my calculation is pretty slapdash. But I figure that a
two-data-structure solution might have even worse locality. And, of course, I
was comparing the linear search against a hypothetical hashed search that took
zero time.

I think the rule of thumb about linear versus hash is that tables smaller than
10--15 entries are faster or the same with a linear search. So I should
calculate some additional statistics about the fields per record.

On the other hand, I'm always up for timing hopeful optimizations, so I may
go ahead and implement the MapVector approach.