Add support for TREAT operator

[beikov]

JPA 2.1 introduced the TREAT operator and we should add support for that in the parser and the API.

[beikov]

Some issues that we should keep in mind

https://hibernate.atlassian.net/browse/HHH-9345
https://hibernate.atlassian.net/browse/HHH-9342
https://hibernate.atlassian.net/browse/HHH-8974

[beikov]

Treat is allowed to be used as

• join path expression
• normal path expression

We have to reflect that in the grammar

[beikov]

I created a testsuite for treat operator testing and started dicussions in the forums of EclipseLink and Hibernate. Let's see what they say before we start implementing something.

[beikov]

I implemented rudimental treat support in the core: 1d3fd3c
Since hibernate and datanucleus implement implicit subtype property resolving, most cases can work without rendering the treat through to the final query.
The edge cases described in the testsuite could actually be fixed or at least be handled in a deterministic way, but I'd rather wait for a request for it or a statement from the EclipeLink or Hibernate community regarding the treat issues.

[beikov]

Also note that #123 contains the explanation of a workaround for some treat limitations.

[beikov]

• Integrate tests from the jpa-treat-variations repository
• Implement workarounds for making tests work Deferring as Hibernate can handle all cases
• Implement criteria support
• Documentation
• Create tests that use a TREAT as fetch parent for fetchOnly()

Notes for possible workaround implementations

• We could introduce virtual predicates that contain TREAT expressions for other subtypes to workaround premature subtype filtering issues

Notes about limitations

• EclipseLink does not support further joining on a treated join node: https://bugs.eclipse.org/bugs/show_bug.cgi?id=512870