Make C structs and unions inherits from Struct. Make it work so you can invoke `to_s` on them (for unions it just prints the name of the type).

I believe it is important to keep the names consistent everywhere, even
the tests. If our direction moving forward is to use the LibX
convention, then that should be reflected everywhere.

Now function types inherit the Function generic variadic struct, so you can do Function(Int32, Int32), which is the same type as Int32 -> Int32. This makes the language more consistent, but also allows using "Function" as a type restriction.