A lot of isinstance checks no longer did anything, since the referred to variable was always a type. In some places, we used it to check if a type was internal, it's unclear if we will need to rebuild those checks in the future.
They were exposed on AST, causing confusion. Now they're only used in constraints.
- Make struct into a type constuctor - Rework placeholders - Got rid of 'PrimitiveType' as a concept - Moved out the prelude to its own folder