Dataset.filter doesn't count removed items

[zhiltsov-max]

Since #102, Dataset.filter() and other transform operations can produce dataset patches, which include a list of removed items. However, they do not count items removed by transforms (and filters), only direct removal with .remove() is counted.

How to reproduce:

1. Create a Dataset
2. Call Dataset.filter / Dataset.transform, which excludes some items
3. Check Dataset.patch

Dataset.patch is expected to have some items marked removed.