Abstract polymorphic type definition to allow non-contenttype scenarios

[charettes]

One should be able to define the type of an object based on another kind of field.

For example:

1. The first two characters of a string primary key might identify the type of the object;
2. An additional field (say tableoid in the case of PostgreSQL inheritance) could identify the type of the object.

[charettes]

It's only missing tests and docs at this point.