Also change some code formatting.
lint::builtin becomes a sibling of lint::context in order to ensure that lints
implemented there use the same public API as lint plugins.
The immediate benefits are
* moving the state specific to a single lint out of Context, and
* replacing the soup of function calls in the Visitor impl with
more structured control flow
But this is also a step towards loadable lints.
We're going to have more modules under lint, and the paths get unwieldy. We
also plan to have lints run at multiple points in the compilation pipeline.
