Move validation before the lowering stage

[mhasel]

Moving the validation step before lowering the user-written AST would remove the need for multiple safe-guards in the lowering-stage. Additionally, it would ensure only user-written code is validated. The drawback of this is that it is no longer possible to call the generated init-functions in code directly, since at the time of validation they cannot be resolved - this is problematic when not working with a runtime.

Since we are working on eliminating the need for calling init-functions manually by adding them to the .init_array, we could re-evaluate the order of execution of these pipeline-stages.