[C++] (Possibly) Adopt unordered_map replacement

[pitrou]

Describe the enhancement requested

We use unordered_map quite a bit internally. This can have performance issues if the hash table grows large. We may want to replace those uses with a faster associative container that would have better performance and/or a smaller memory footprint.

This pages gives useful pointers: https://martin.ankerl.com/2022/08/27/hashmap-bench-01/

Caveats:

1. the replacement must be easily vendorable, with no additional dependencies (this excludes abseil, folly, etc.)
2. the replacement should be maintained
3. we probably don't want to replace uses of unordered_map in public APIs with a non-standard container type

From the looks of it, https://github.com/martinus/unordered_dense might be a good candidate.

Related: #25111

Component(s)

C++