Enable codegen-units = 1 optimization for the Rust part

Hi!

I see that in the Cargo.toml [file](https://github.com/deepseek-ai/3FS/blob/main/Cargo.toml) you've already enabled LTO optimization - that's great! I suggest you additionally enable `codegen-units = 1` [optimization](https://doc.rust-lang.org/rustc/codegen-options/index.html#codegen-units) - it will lead to better compiler optimization opportunities that consequently leads to smaller binary size and better performance.

Thanks!