v2.0.0

This is a major release.

1. Major optimizations and refactoring.
2. Rewritten pre-validation modules.
3. Element aggregations now can be used in member paths.
4. Validation of heterogeneous containers.
5. Validation of trees.
6. Support of value transformers.
7. Variadic properties with arbitrary number of arguments.
8. Nested validators.
9. Validation of pointers.