Swift 4 templates

[djbe]

As mentioned in SwiftGen/SwiftGen#312 by @csnu17, there apparently are some issues with the current templates on swift 4.

We should fix these errors(*), and if possible, use new language features to simplify the templates where needed.

(*) more accurately, create a new swift4 template—based on the swft3 templates but with fixes to make it compile to Swift 4— so that we'll keep both swift3 and swift4 templates for the users of SwiftGen to choose from

[AliSoftware]

There's currently a note in the README mentioning that "we don't have Swift 4 templates yet but they are coming soon"; reminder that we should obviously not forget to remove that note once this issue is implemented.

[djbe]

@csnu17 SwiftGen 5 was released today. Could you provide some sample code where the current swift 3 template fails when using swift 4 code? From cursory testing in playgrounds, everything seems to work for me.