There is now a CoreEmitter that everything desugars to, but without
losing any information. Also remove RenderSpan::FileLine. This lets the
rustc_driver tests build.
Major changes:
- Remove old snippet rendering code and use the new stuff.
- Introduce `span_label` method to add a label
- Remove EndSpan mode and replace with a fn to get the last
character of a span.
- Stop using `Option<MultiSpan>` and just use an empty `MultiSpan`
- and probably a bunch of other stuff :)
This uncovered a lot of bugs in compiletest and also some shortcomings
of our existing JSON output. We had to add information to the JSON
output, such as suggested text and macro backtraces. We also had to fix
various bugs in the existing tests.
Joint work with jntrnr.
Automated conversion using the untry tool [1] and the following command:
```
$ find -name '*.rs' -type f | xargs untry
```
at the root of the Rust repo.
[1]: https://github.com/japaric/untry
