MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
@zdne On re-review, I think you can leave it. It does not collide with other ideas in the AST albeit it coincides with Nested Type Name List in the spec. I think for simplicity, `nestedTypes` is fine. Or possibly, `nestedTypeList`. 

However, this list is only applicable to `array` and `enum` inheriting structures, so you may want to put something to that effect in the inline description about it.