add support for Fn::Split

[bpholt]

Resolves #140.

Because Fn::Split logically returns Seq[Token[String]], I wanted to make sure it could be used anywhere Seq[Token[String]] is used today. That's why I added the TokenSeq[R] (which is a type alias for an Either): either the caller can use a hard-coded sequence of strings (or Token[String]s), or they can use Fn::Split. In the first case, the resulting JSON is a JsArray, and in the second, the resulting JSON is a JsObject. Either way, the Either should be fairly transparent to the caller because there are implicit functions that lift values into the Left or Right as appropriate.

[bkrodgers]

Thanks for the submission! So is your expectation that with the implicits you've added that this would be fully backwards compatible with existing templates? If so I'll test it out on a few of our existing templates to confirm.

[bkrodgers]

I built this and tried it on one of our old templates. Seems to have worked fine with no changes to the template.

@tj-corrigan , could you give this a look too before I merge it?

[bpholt]

Glad it works with your templates! It was my intention that the implicits would make it backwards-compatible. It seems to be so as far as I could tell, but we don't use all the AWS products this PR touched, so I couldn't be 100% sure.

[bkrodgers]

TJ hasn't had time to look at this, but I looked at it again myself and I think we're good to go. Merging.