002: String id_ support
In version 0.3.0 we dropped support for string ids, and that was a bad decision, as there are already projects that use it.
The reason of deprecation was because the new feature to auto increment the ID of the entities that hadn't set it, wasn't "easy" to implement for strings.
Restore the support for string ids, but without the auto increment feature. If you're using string ids is probably because you have an id generator or you're getting them from outside sources.