Streaming bulk load API

[marvin-j97]

Possible API

// tree needs to be empty
//
// writer wraps MultiWriter
let mut writer = tree.ingest()?;

// items needs to be sorted ascending
for item in items {
  writer.write(item.k, item.v)?;
}

// commit needs to register all segments into the last (?) level
writer.commit()?;

Should also work for blob tree.

Possible API consideration: hash all inputs and allow verifying import was 100% correct (paranoid integrity check).