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ARROW-8658: [C++][Dataset] Implement subtree pruning for FileSystemDataset

TODO:
- [ ] Add benchmarks to be sure this provides an advantage
- [ ] Move Forest to dataset_internal.h or so, since it is not used anywhere else
- [ ] Unit test SubtreeImpl, add more comments

Closes #9670 from bkietz/8658-Implement-subtree-pruning

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>