Provides a Ceylonic [[EntityManager]] for use with the Java
Persistence API. An `EntityManager` may be instantiated
directly, given an instance of
[[javax.persistence.EntityManager|javax.persistence::EntityManager]].
value emf = Persistence.createEntityManagerFactory("example");
value em = EntityManger(emf.createEntityManager());
The `EntityManager` provides all the same operations as
`javax.persistence.EntityManager`, but:
1. is slightly more typesafe, and
2. accepts [[Integer]], ...
