serializable types as basic types

Since 1.0, the Persistence spec has said that any un-annotated field is basic by default, and also provided a fall-through to Java serialization for unrecognized basic types.

This is a misfeature.

- Storing serialized Java objects in the database is almost always a rubbish thing to do.
- The fall-through prevents the provider from providing meaningful error reports for unrecognized types, or when mapping annotations are missing.
- In case a user *really* wants this, it's quite straightforward to provide an `AttributeConverter`.

So I think we should at least deprecate this, and perhaps even consider having some way to just turn this "feature" off.

Thoughts?

Start by reviewing the Persistence spec sections covering un-annotated fields, basic types, Java serialization, and AttributeConverter. The issue does not name implementation files or tests, and its open question is whether to deprecate serialization or provide a way to disable it. Done would require a decided scope and corresponding specification and compatibility work.

Written by the indexing model from the issue text.