Function chain_extend 

pub fn chain_extend<H: Hash>(prev_chain: &H::Out, new_image: &Image) -> H::Out
where
    H::Out: AsRef<[u8]>,

Extend an image-hash chain with a new image: result = H(prev_chain || image_content_hash(new_image)).

Used for both set_image(new) on an existing Instance and host_derive_spawn(new, cnode) from a spawner.