explore possibility of removing older C libraries

[clux]

Some of the C libraries in here are potentially less necessary down the line as rust alternatives gain complete majority and stability.

Only two candidates for removal

curl

As far as I can there are very few uses of curl variants these days (it was more common pre-hyper dominance), but the curl crate still has plenty of downloads.

zlib

The flate2 crate can possibly replace zlib as it bundles miniz.c as the default implementation.
But, don't think it works with pq, and the openssl build probably also benefits from it (it might disable gzip support otherwise).

No work planned

This is mostly a tracking issue for me. No immediate action is planned here this year. If you use these libraries and would like them to stay, or otherwise have input on this, please leave a comment.