Add constant generation for Swift, Scala and Kotlin

[edif2008]

As part of #218 we've added support for integers constants. Currently this only works for Go, Python and Javascript. We want to expand the capability to generate constants on the other supported languages.

The overall framework for parsing and generating constants is in place. All that's left is to add the specific code that needs to be generated for the following languages: Swift, Scala, Kotlin.