OCL25 — Clarify xxxEval classes

OCL25-80 comment 1 identifies that the xxxValue classes are misleading and could be trivially rephrased as "xxx value semantics".

Similarly the xxxEval classes are unhelpful and could be triially rephrased as "xxx evaluation semantics".

Simple or efficient implementations of OCL have no need to reify each evaluation as an instance, the execution can just return the appropriate result. Only an implementation with rigorous tracing for debugging or incremental execution need reify the execution and the implementation should be free to choose its own implementation details.

There is absolutely no need for a normative existence of xxxEval classes. Only the semantics is normative.