fix some clippy warnings, use PhantomData for the lifetime marker in Iter and IterMut

[tsemo4917]

fix some clippy warnings

• 1 redundant clone because Option<&usize> which implements the Copy trait
• some style issues（GlobalAlloc is unused when "default-features = false"）
• 1 clippy::manual_bits 8 * (size_of::()) -> usize::BITS as usize

feat: use marker for the lifetime marker in Iter and IterMut

• remove the allow(dead_code) and reduce size of Iter and IterMut, without any compiler warnings
• PhantomData: https://doc.rust-lang.org/core/marker/struct.PhantomData.html