perf: reduce llvm-lines by removing .join(", ")

[dishmaker]

• Related to perf(toml): Rewrite validate_struct_keys: remove .join() toml-rs/toml#1059

rustc 1.93.0-nightly

Before:
syn$ cargo llvm-lines --release | head -n 3
  Lines                 Copies              Function name
  -----                 ------              -------------
  113775                2809                (TOTAL)

After:
syn$ cargo llvm-lines --release | head -n 3
  Lines                 Copies              Function name
  -----                 ------              -------------
  112105                2793                (TOTAL)

[dtolnay]

Thanks!