Refactor Include evaluator. Implement caching.

[fiseni]

We're storing the Include expressions as LambdaExpression in the specification. We have no other choice, we can not store the sub-chains in generic form.

Then, in the evaluator, we call the EF generic methods through reflection. To optimize the reflection calls, we have a cached implementation where we create and cache proxy delegates. This improves performance significantly. Currently, this implementation is an opt-in feature.

I think we should finally make this implementation the default one. We need to clean it up a bit and make some further optimizations.

Breaking Changes:

• The common use of this library will remain unchanged.
• The SpecificationEvaluator no longer accepts bool cacheEnabled parameter.
• The include string evaluation is extracted in a separate IncludeStringEvaluator construct.
• The IncludeEvaluator.Default and IncludeEvaluator.Cached singleton instances are removed. Instead, use the IncludeEvaluator.Instance.