Implement SelectMany support

[amdavie]

For issue #294

Usage:

public class StoreProductNamesSpec : Specification<Store, string?>
{
  public StoreProductNamesSpec()
  {
    Query.SelectMany(s => s.Products.Select(p => p.Name));
  }
}

Concurrent usage of Select and SelectMany is prevented via a runtime evaluator exception.

We may want to rename ISpecification<T, TResult>.Selector to SelectExpression to match the newly added SelectManyExpression to be consistent (I couldn't come up with a good name other than SelectManyExpression).

Let me know how it looks - happy to make additional changes.

[fiseni]

Hey @amdavie

Thank you for your work.
I don't like the Selector name either, but we've defined that since the beginning, and would be a breaking change to rename it. Therefore, let's name the other one SelectorMany perhaps? Just to be consistent.

[amdavie]

@fiseni no problem. SelectorMany it is.