function loadChanges(backend, changes) {
  const state = backendState(backend)
  state.applyChanges(changes)
  backend.frozen = true
  return {state, heads: state.heads}
}
Applies a list of `changes` to the node state `backend`, and returns the updated
state with those changes incorporated. Unlike `applyChanges()`, this function
does not produce a patch describing the incremental modifications, making it
a little faster when loading a document from disk. When all the changes have
been loaded, you can use `getPatch()` to construct the latest document state.