Move state (fields) from Mock into Mock<T>

[stakx]

Mock state and initialization is currently split across both Mock and Mock<T>. This is not ideal for at least three reasons:

1. It is difficult to get an idea how much state a mock object really carries.

2. It can be hard to understand control flow (e. g. with mock object initialization), which might bounce back and forth between methods defined in both Mock and Mock<T>.

3. It's relatively easy to introduce As<TInterface>-related bugs by directly referencing backing fields instead of their corresponding properties. As<TInterface> mocks do not use their own backing fields (which remain completely uninitialized), but instead delegate all properties to the "owner" mock's. Therefore, for As<TInterface> mocks to work correctly, code should always reference the properties, and never the backing fields. Moving the fields away from Mock (where most logic sits) helps with enforcing this.