[Feature Request] Remove dependence on `abseil-cpp`

Required prerequisites

[X] I have searched the Issue Tracker that this hasn't already been reported. (comment there if it has.)

Motivation

abseil-cpp is a collection of useful C++ utilities, that make our code well-organized and more readable. However, it poses some packaging challenges when we need to support various C++ compilers and their standard libraries (especially for conda-forge packages).

See also:

80
conda-forge/optree-feedstock#18
https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=782866&view=logs&j=9c5ef928-2cd6-52e5-dbe6-9d173a7d951b&t=20c71c51-4b27-578b-485d-06ade2de1d00

Current references:

$ grep -r -h -o -i -E 'absl::\w+|absl\w+' include src | sort | uniq
AbslHashValue
AbslHashValueImpl
absl::HashOf
absl::InlinedVector
absl::Span
absl::StrAppendFormat
absl::StrCat
absl::StrFormat
absl::StrJoin
absl::c_copy
absl::flat_hash_map
absl::flat_hash_set
absl::str_format

Solution

Remove the dependence on abseil-cpp and move to STL libraries.

Alternatives

No response

Additional context