arrow

ARROW-5336: [C++] Implement arrow::Concatenate for dictionary-encoded arrays with unequal dictionaries

The dictionaries still need to have the same index & value types.  It is possible that concatenating two dictionaries still fails because the resulting dictionary has more values than its index type can represent.

The unification will still fail if nulls are present in either dictionary.  The canonical approach seems to be representing nulls in the indices array with a validity bitmap.  The existing unifier had this constraint in place.  My guess is that this was to avoid making the memo table null-aware.  It could be handled without modification to the memo table by using a -1 index and so I could easily add this if desired.  I wasn't sure if support for this non-typical case justified the complexity.

Closes #8984 from westonpace/feature/arrow-5336

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>