Region::compute_stats: use temporaries?

[yakra]

Floating-point division is expensive. Some redundant calculations occur not just for a HighwaySegment itself, but every traveler on it.

const double act_length = s.length/act_concurrency_count;
const double a_p_length = s.length/a_p_concurrency_count;
const double sys_length = s.length/sys_concurrency_count;

and replace the expressions accordingly.

IIRC, benchmarking during the Region::compute_stats reunification (TravelMapping#584) suggested this didn't amount to much, but it may be worth another look.

But first?

• [ ] Would a change to std::map for the per-user regional tables make things faster or slower? Would changes from this optimization be easier to see?
• [ ] "ComItOpt" redux: Further iterator optimization via __builtin_ctz (see TravelMapping#598)