Function write 

pub fn write(
    into: &mut dyn Backing,
    at: Rev,
    document: &Document,
    payloads: &mut dyn AssetSink,
) -> Result<Digest>

Write document as rev at and hand back the digest of the bytes that landed — which is what the manifest row stamps, and the only state digest this format has.

The payloads go to assets/ first: a rev may not name bytes the store does not already hold, and every payload the document holds is put there, not only the ones it currently references, so a payload outlives the reference that brought it in.

Errors

The write that did not land.