Split up lint/builtin.rs

[kmcallister]

It's getting pretty big, and the code is very modular.

[huonw]

It would be neat to move as many of these into an external crate as possible, to enable faster iteration.

[richo]

It seems like this can be closed, unless I'm misinterpreting the issue to actually split each lint out into it's own crate.

[caipre]

@kmcallister / @huonw close?

[huonw]

Most lints are now defined in the librustc_lint crate, but they're all in one huge file. It would make sense to break that file up (e.g. an "obvious" division would be breaking out separate modules for the lint groups).