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ARROW-7148: [C++][Dataset] Major API cleanup

- Make use of `Result<T>` instead of Status/output-parameter (fixes ARROW-7147).
- Make use of type forward aliases, e.g. the KlassPtr pattern. This ensure a normalization of the correct pointer type and a shorter types to write.
- Use std::shared_ptr everywhere for normalization.
- Move implicit filter cast into R binding (fixes ARROW-7188).
- Fix type validation issue.
- Force FileSystemDataSource to use an owning pointer for the filesystem (fixes ARROW-7094).
- Make constructors private.
- Delete not implemented format header, i.e. CSV, JSON and Feather.
- Delete unused classes ProjectedRecordBatchReader.
- Fold SimplerScanner into Scanner.
- Rename FileBasedDataFragment to FileDataFragment and
  FileSystemBasedDataSource to FileSystemDataSource.

Closes #5857 from fsaintjacques/ARROW-7147-refactor-dataset-result and squashes the following commits:

aa3b74bc3 <François Saint-Jacques> Fix R test
53a7454bf <François Saint-Jacques> Address comments
ec00d1d8a <François Saint-Jacques> ARROW-7147:  Major API cleanup

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>