versions in prototype chain

proto-version.js

// IDEA: start with an object using a symbol as a marker
const create = (obj, changeSet) => Object.assign(Object.create(obj || {[Symbol.for('versionstart')]: true}), changeSet);

const getPreviousState = obj => {
  const maybePrev = Object.getPrototypeOf(obj);
  if (!maybePrev || maybePrev[Symbol.for('versionstart')]) {
    return null;
  }
  return maybePrev;
}

// TODO how do we know how many versions we have?