p521: apply clippy manual_swap lint

[dishmaker]

$ cargo clippy --version
clippy 0.1.101 (c36f145719 2026-10-01)

warning: this looks like you are swapping elements of `ret` manually
   --> p521/src/arithmetic/field.rs:191:13
    |
191 | /             let tmp = ret[i];
192 | |             ret[i] = ret[j];
193 | |             ret[j] = tmp;
    | |_________________________^ help: try: `ret.swap(i, j);`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/main/index.html#manual_swap
    = note: `#[warn(clippy::manual_swap)]` on by default