MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
**More clarity**: In `Named Type`, we have just an array of `sections` of type `array[Type Section]`. but since `Type Section` can represent `description`, it's good. Here, we have both `description` and `sections` which is, well, a bit redundant. I am saying, we can probably remove `description` here and it would still be correct since `sections` represent `description`. And we should do that.