Batch of improvements to errors for new error format
This is a batch of improvements to existing errors to help get the most out of the new error format.
* Added labels to primary spans (^^^) for a set of errors that didn't currently have them
* Highlight the source blue under the secondary notes for better readability
* Move some of the "Note:" into secondary spans+labels
* Fix span_label to take &mut instead, which makes it work the same as other methods in that set
MultiSpan model is now:
- set of primary spans
- set of span+label pairs
Primary spans render with `^^^`, secondary spans with `---`.
Labels are placed next to the `^^^` or `---` marker as appropriate.