rename crate to kzg-bn254

[samlaf]

It's redundant and atypical to have a rust crate be named rust_.... We should probably just rename it to kzg-bn254.

Not sure what the implications are wrt publishing a new crate. Does crates.io easily allow deprecating a crate and pointing to a new one? What version would we start the new one at? etc.

Could also consider renaming repo to kzg-bn254-rs, which feels more idiomatic than a rust- prefix.