Collect various one-off analysis tidbits into a 'lint' pass

[brson]

I don't really feel that little things like #1539 should be buried in semi-arbitrary passes that have other important responsibilities. These little nice-to-have checks should probably be corralled somewhere where they can be switched on and off in an orderly way.

Edit: changed pass name to "lint", as discussed on IRC

[lht]

@brson Please review if 460a81a implemented what you meant?

I feel there should be a better name for the pass and option, but I cannot come up one. Please suggest if you have a better idea.

[lht]

Also as discussed on IRC, a crate attribute will be added for switching the various one-off analyses. For example:
#[lint(ctypes, unused-imports)]

[graydon]

We discussed this a bit during a meeting today, in terms of the ability to control warnings and errors by attribute. I think the consensus was to make most warnings-and-errors controllable via an attribute, perhaps #[warn(...)] and #[error(...)]? If we do this, I don't mind keeping the lint source-module for "miscellaneous checks" but perhaps we can remove the lint attribute in favour of the warn and error ones?

[brson]

Punting on this for 0.2

[lht]

Assigned to myself. Looks like I'll finally find some time work on Rust. :)

[catamorphism]

As far as I can tell, this is finished? If it's not, please open more specific individual bugs.