Function concatenate_info 

fn concatenate_info<F, R>(info: &[&[u8]], f: F) -> R
where
    F: FnOnce(&[u8]) -> R,

Concatenates info slices into a contiguous buffer for HKDF operations. Uses stack allocation for typical cases, heap allocation for large info. Info is public context data per RFC 5869, so no zeroization is needed.