[Plat-8841] feature flags order

[kstenerud]

Goal

PLAT-8841: Keep ordering of feature flags, and improve performance of the feature flags store

Design

Replaced the array based feature flags store with a combined array and dictionary in order to speed up access.

Each dictionary entry maps to an index in the backing array, which contains the feature flag. Removing a feature flag leaves a hole in the array in order to avoid compaction costs. A forced compaction occurs only if there are more than 1000 holes.

The measured performance difference between this and a straight-up unordered dictionary is negligible (I've put in performance tests for easy comparison).

Testing

Updated tests to match the new expected feature flag ordering, and added some performance tests.

[github-actions]

Bugsnag.framework binary size increased by 2,760 bytes from 702,608 to 705,368

    FILE SIZE        VM SIZE    
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  +0.4% +2.70Ki  +2.3% +16.0Ki    TOTAL

Generated by 🚫 Danger

[nickdowell]

Implementation looks sound but currently too much is exposed in BugsnagInternals.h and I'm hoping BSG_OBJC_DIRECT_MEMBERS will help reduce the bloat (currently +3.4K 🙁)