code: function followEntryRefs( key, callback ) {
     return function( entry ) {
        const refs = entry[ key ] || [];
        return Promise.all( refs.map( callback ) ).then( flatten );
     };
  }
docs: Create a function that expects an artifact entry as a parameter and follows the given key of it with
  a given function.
  
  @private
  @param {String} key the key to follow
  @param {Function} callback the function to call for each ref listed in the key
  @return {Function} a function returning a promise for a list of entries returned by callback