Consider adding `classify`.

[Ignat Insarov]

Hello.

There has been [a question on Stack Overflow][1] asking for a way to group a
list by an equivalence relation. Several answers were proposed over time, and I
too [have offered a variant][2]. [I also wrote a benchmark.][3]

I propose that the function be added to the standard libraries.

[1]: https://stackoverflow.com/q/8262179
[2]: https://stackoverflow.com/a/57761458
[3]: https://github.com/kindaro/classify-benchmark.git