arrow

ARROW-6883: [C++][Python] Allow writing dictionary deltas

* Add an ipc::IpcWriteOptions member to govern emission of dictionary deltas.

  If the option is enabled, deltas are detected by checking whether the new
  dictionary starts with the last emitted one for the same field.
  However, for nested dictionaries, deltas are not emitted for the outer dictionary,
  as the read path doesn't support it.

* Add a stats() method to ipc::StreamDecoder

* Expose the IPC statistics in Python, and add tests

Closes #8811 from pitrou/ARROW-6883-ipc-write-deltas

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Wes McKinney <wesm@apache.org>