[WIP] [C++] DenseUnionArray

[pcmoritz]

This is work in progress, a first pass at implementing DenseUnionArray for Arrow. It is ported over from ray-project@0707062.

[wesm]

ARROW-373 contained a bit of refactoring so you'll need to rebase, sorry about that

[pcmoritz]

Thanks for letting me know! Going forward, do you want to have only one union array class and make it dense or sparse depending on the type field?

[wesm]

that seems acceptable for now, we may want dedicated container classes at some point, though.

[wesm]

@pcmoritz how would you like to proceed? In the course of implementing unions, we'll also want to add integration tests once ARROW-394 and associated JIRAs are in

[pcmoritz]

@wesm Let's try to get this in a form where it is mergeable, this means making the equals function work and having some more tests on the C++ side. I can't yet promise how soon I'll get to that, I hope I'll have time next week. I'm happy if somebody continues on it earlier if this is blocking.

We also should make sure that we have some specification of the binary format so Java and C++ use the same format. I'll probably not look into the Java side any time soon though.

[wesm]

@pcmoritz this is specified in https://github.com/apache/arrow/blob/master/java/vector/src/main/java/org/apache/arrow/vector/schema/TypeLayout.java#L67

[wesm]

Since I've done a lot of work recently on the IPC code and integration testing and have that all fresh in my head, I'll go ahead and do the union implementation in the next week or two. Will report back