Rename lint::Lint to lint::LintId
Convert lints to a trait-based infrastructure

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.
Replace enum LintId with an extensible alternative

Temporarily eliminate -W help in the process.
wip