refactor: apply clippy::explicit_auto_deref

[shulaoda]

Summary

• Removes redundant explicit deref/borrow patterns (&*x, &**x, &***x) across src/expr.rs, src/items.rs, src/reorder.rs, src/types.rs, and src/visitor.rs. The compiler's auto-deref coercion produces the same reference type.
• Addresses clippy::explicit_auto_deref, run as a single-rule pass: cargo clippy -- -A clippy::all -W clippy::explicit_auto_deref.

Representative changes:

• contains_skip(&*expr.attrs) → contains_skip(&expr.attrs)
• rewrite_index(&**expr, &**index, …) → rewrite_index(expr, index, …)
• is_same_item_kind(&***ppi) → is_same_item_kind(ppi)
• result.push_str(&*format_visibility(…)) → result.push_str(&format_visibility(…))

[ytmimi]

Going to close until we decide on which clippy lints we want to enforce in the codebase.