warn about unused variables from patterns

[jruderman]

use std;
tag t { alpha; beta; }
fn f(x : t) { alt x { alfa { } }; }
fn main() { }

Should warn that "alfa" is an unused variable. (I misspelled the tag name and forgot a period; the warning could help me notice that I didn't succeed in having it treated as a tag name.)

[Yoric]

I will try and work on that.

[Yoric]

Finding my way around the typing code is harder than expected. I will try and return to the charge in a few days.

[catamorphism]

Still doesn't warn (7/26/2012).

[nikomatsakis]

Fixed as of e47d2f6.