Improve use of generics and privacy levels in `ArraySchema` and `MapSchema`

[rnd256]

• ArraySchema's method signatures for GetItems(), GetByIndex(index), ContainsKey(key), GetTypeDefaultValue(), SetItems(), and InvokeOn{Add,Change,Remove}() should use type T.
• The Items member should probably be private.
• It seems like there are some methods that manage internal details which shouldn't be public (eg. GetIndex(index) without the By in the name, and anything that has dynamicIndex in its signature).
• Similar improvements can be made to MapSchema.

[endel]

Thanks for the feedback @rnd256, I agree completely, all these methods should be for internal usage only and not exposed. I hope we can tackle this sooner or later.

[rnd256]

I think the methods in the first bullet point I mention should remain public, but agreed on making the other ones private.