Preserve parentheses around `Fn` trait bounds in pretty printer The AST pretty printer was dropping parentheses around `Fn` trait bounds in `dyn`/`impl` types when additional `+` bounds were present. For example: dyn (FnMut(&mut T) -> &mut dyn ResourceLimiter) + Send + Sync was pretty-printed as: dyn FnMut(&mut T) -> &mut dyn ResourceLimiter + Send + Sync Without parens, `+ Send + Sync` binds to the inner `dyn ResourceLimiter` instead of the outer type, producing invalid Rust. The parser already tracks parentheses via `PolyTraitRef.parens`, but `print_poly_trait_ref` never checked this field. This adds `popen()` and `pclose()` calls when `parens == Parens::Yes`.