Remove the state for the Query builder. Update it as a calculated property.

[fiseni]

We no longer will keep the specification builder a state. It will be defined as a calculated property and we'll create an instance on access public ISpecificationBuilder<T> Query => new SpecificationBuilder<T>(this);

In version 9 we'll improve the builders, so the whole query can be written as a single chain. Therefore the common use will not require more than one allocation. By removing the state, we'll save an additional 8 bytes used for the reference.

Other than that, the property will no longer be defined as virtual. Not a single user is overriding and defining their specification builder. The builder is the essence of this library.