[model generation] Remove the `metadata` argument from pure functions

As discussed with @bugarela, we need to remove the unnecessary `metadata` argument and switch to the new stub format.

* Remove `deps` (from the state and from any arguments for pure functions). We are not using it.
* Pass `msgInfo` and `env` as-is (nested records).
* Figure out a better UI so that users can enter flat structures instead of nested records.