Reorder the diagnostic API methods.

The current order is almost perfectly random. This commit puts them into
a predictable order in their own impl block, going from the highest
level (`Block`) to the lowest (`Expect`). Within each level this is the
order:

- struct_err, err
- struct_span_err, span_err
- create_err, emit_err

The first one in each pair creates a diagnostic, the second one creates
*and* emits a diagnostic. Not every method is present for every level.

The diff is messy, but other than moving methods around, the only thing
it does is create the new `impl DiagCtxt` block with its own comment.

Author: nnethercote