Simplify diagnostics in the core type checker

[Brendan Zabarauskas]

I think it would be fine to simplify reporting::CoreTypingMessage in order to reduce the friction involved in adding new features. Seeing as it is not user-facing I think it would be fine to simplify it to something like:

struct CoreTypingMessage {
    file_id: usize,
    range: Range<usize>,
    message: String,
}

[Brendan Zabarauskas]

As of #296, the core type checker has been removed (for now).