Use official abseil-cpp

[taku910]

We are currently using a compatibility subset of the Abseil code, but we would like to replace it with the official package. This is expected to reduce maintenance costs and also improve performance. For example, the benefits of using absl::flat_hash_map are significant

[Varshith2403315]

Hi, I’m a first-time contributor and I’d like to help with this migration.

From what I understand, the project currently uses a compatibility subset of Abseil, and the goal is to replace it with the official abseil-cpp dependency (e.g., using absl::flat_hash_map directly) to reduce maintenance and improve performance.

To keep the change scoped, I’d like to start by:

Identifying where the current Abseil compatibility code is used
Replacing one well-defined component (for example, a specific map/set usage) with the official absl equivalent
Updating build configuration as needed for that part

Does that sound reasonable, or is there a specific sub-area you’d prefer me to work on?