Add Fheanor

[FeanorTheElf]

It would be great if Fheanor could be added to the list. It's an FHE library written in Rust, with the goal of making research on variants of FHE schemes easier.

How this meets the guidelines for contribution:

• We uploaded an accompanying paper to eprint https://ia.cr/2025/864 a few days ago, but the library has been in development for more than a year (previously being called HE-Ring).
• In terms of implemented features, the project has some similarity with HElib and other BGV/BFV libraries, but has different design goals.
• Adding Fheanor has not been proposed previously.
• Fheanor has an extensive documentation, available on https://docs.rs/fheanor/latest/fheanor/