ARROW-460: [C++] JSON read/write for dictionaries

[wesm]

cc @BryanCutler; this ended up being rather tedious.

As one uncertainty: I wasn't sure what to write for the typeLayout field for dictionary fields. We don't use this at all in C++, so let me know what you need in Java (if anything)

[BryanCutler]

Since typeLayout is optional in Java, you don't need to write anything and it will default to a 32bit Int. I believe this field is for specifying an Int of a different bitwidth - so does C++ just use a 32bit index?

[wesm]

We don't use the typeLayout at all on read; we use the indexType from the dictionary metadata

[wesm]

OK, dictionaries are now in the top level. here's an example json:

https://gist.github.com/wesm/5100e41173a3b5e53437b7a887e4383a#file-gistfile1-json

[wesm]

As you can see in the JSON, the typeLayout I am writing is the index type

[wesm]

+1. Merging this and we can reconcile JSON format discrepancies when the Java version is complete and we add the integration tests themselves

[BryanCutler]

Sorry @wesm , I got mixed up between indexType and typeLayout in my response. Java expects the encoded Field to have a typeLayout to describe the dictionary type so it can then proceed to read the dictionary data. How do you that in C++ if that metadata isn't there?

[BryanCutler]

I'll post my PR soon, there is still a couple things that need to be worked out, but it can read/write JSON so we can start testing.

[wesm]

The typeLayout right now is deterministic based on the other type metadata, so we don't need it. I can change the typeLayout to be the dictionary type layout if it causes a problem