Port lint.rs from oldvisit to <V:Visit> trait API. Less mechanical port.

That is, there was lots more hacking than the other more-mechanical
ports Felix did.

There's also a strange pattern that I hacked in to accommodate the
Outer/Inner traversal structure of the existing code (which was
previously encoding this by untying the Y-combinator style knot of the
vtable, and then retying it but superimposing new methods that "stop
at items").  I hope either I or someone else can come back in the
future and replace this ugliness with something more natural.

Added boilerplate macro; all the OuterLint definitions are the same
(but must be abstracted over implementing struct, thus the macro).

Revised lint.rs use declarations to make ast references explicit.
Also removed unused imports.