perf: use flat_set, flat_map for small, trivially-moved containers

[ckerr]

Description of Change

base::flat_map and base::flat_set are STL-like map & set containers that are implemented using std::vector instead of an red-black tree, so they have good memory locality and lower overhead, especially for containers with smaller sizes. The upstream usage advice recommends:

Most maps and sets in Chrome are small and contain objects that can be moved efficiently. In this case, consider base::flat_map and base::flat_set.

You need to be aware of the maximum expected size of the container since individual inserts and deletes are O(n), giving O(n^2) construction time for the entire map. But because it avoids mallocs in most cases, inserts are better or comparable to other containers even for several dozen items, and efficiently-moved types are unlikely to have performance problems for most cases until you have hundreds of items. If your container can be constructed in one shot, the constructor from vector gives O(n log n) construction times and it should be strictly better than a std::map.

This PR replaces std::map and std::set with their flat counterparts iff they match those recommendations: they are all smaller containers with trivially-moveable types (e.g. ints and pointers).

This is a general cleanup PR so no specific stateholders, but all reviews welcomed

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none

[jkleinsc]

LGTM

[release-clerk]

No Release Notes