feat: Optimizations for itinerary locality and SmallVec for active settings

[RobertJacobsonCDC]

This PR is stacked on top of #89. It introduces two optimizations:

1. It stores a person's SettingIds and ItineraryRatios in a single property instead of in two separate properties. These are often fetched together. This improves data locality.
2. It uses SmallVec for get_active_settings_for_person, avoiding allocating a vector on the heap for this common operation.

Local benchmark results

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  RobertJacobsonCDC_setting_code_value (e364888) vs RobertJacobsonCDC_itinerary_locality
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  Benchmark                                      Base         Current  Change
  ---                                             ---             ---  ---
  init/population/10k                       3.3204 ms       2.9560 ms  -11.0% faster ✓
  init/population/100k                      40.452 ms       37.420 ms  -7.5% faster ✓
  execute/population/10k                    71.617 ms       70.163 ms  -2.0% faster ✓
  execute/population/100k                    1.2048 s        1.1222 s  -6.9% faster ✓

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Keep in mind this is comparing against RobertJacobsonCDC_setting_code_value, not the current main branch. Here's a comparison to the current main:

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  main (9132b89) vs RobertJacobsonCDC_itinerary_locality
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  Benchmark                                      Base         Current  Change
  ---                                             ---             ---  ---
  init/population/10k                       10.994 ms       2.8994 ms  -73.6% faster ✓
  init/population/100k                      113.70 ms       38.173 ms  -66.4% faster ✓
  execute/population/10k                    76.380 ms       70.111 ms  -8.2% faster ✓
  execute/population/100k                    1.1755 s        1.1251 s  -4.3% faster ✓

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[erik-rosenstrom]

Looking good to me!