Use symbols for embed option values for clarify and future compatibility.

Previously `embed: false` would actually embed ids. This confusion cuased
bugs in IDC code for the cache key format, so likely also lead to
misconceptions about what IDC was doing to its users.

Using symbols for values also allows truly normalized cached associations
to be implemented, where the ids for the association are stored in a
different cache value.
Update changelog for unreleased changes.