Better support for complex inheritance

[asm0dey]

The current implementation had several issues:

1. If there are classes A -> B -> C, C will be sealed, A and B will be data. This is incorrect, B should be open.
2. All properties in B and C should be open: descendants can override them
3. If class has no own properties, there can be two cases:
   3.1. It's a leaf (like A), then it should be data object, not just object
   3.2. It's a node (like B), then it should be open class

This commit addresses all these cases.

There is still one important issue I don't immediately know how to fix: all descendants should have an override modifier on fields, overriding fields in the whole tree.