auto merge of #5179 : alexcrichton/rust/default-warn-unused-import, r=graydon

I've found that unused imports can often start cluttering a project after a long time, and it's very useful to keep them under control. I don't like how Go forces a compiler error by default and it can't be changed, but I certainly want to know about them so I think that a warn is a good default.

Now that the `unused_imports` lint option is a bit smarter, I think it's possible to change the default level to warn. This commit also removes all unused imports throughout the compiler and libraries (500+).

The only odd things that I ran into were that some `use` statements had to have `#[cfg(notest)]` or `#[cfg(test)]` based on where they were. The ones with `notest` were mostly in core for modules like `cmp` whereas `cfg(test)` was for tests that weren't part of a normal `mod test` module.

src/test/compile-fail/borrowck-autoref-3261.rs

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use core::either::*;
 enum X = Either<(uint,uint),extern fn()>;
 pub impl &X {
     fn with(blk: fn(x: &Either<(uint,uint),extern fn()>)) {

src/test/compile-fail/borrowck-borrowed-uniq-rvalue.rs

@@ -11,7 +11,6 @@
 //buggy.rs
 extern mod std;
 use std::oldmap::HashMap;
-use std::oldmap;
 
 fn main() {
     let buggy_map :HashMap<uint, &uint> =

src/test/compile-fail/borrowck-move-by-capture.rs

@@ -1,9 +1,5 @@
 extern mod std;
 
-use std::ebml::reader;
-use std::ebml::writer;
-use std::serialize;
-
 fn main() {
     let foo = ~3;
     let _pfoo = &foo;

src/test/compile-fail/issue-2611-3.rs

@@ -13,7 +13,6 @@
 // we let an impl method can have more permissive bounds than the trait
 // method it's implementing, the return type might be less specific than
 // needed. Just punt and make it invariant.
-use core::iter::BaseIter;
 
 trait A {
   fn b<C:Copy + Const,D>(x: C) -> C;

src/test/compile-fail/issue-2611-4.rs

@@ -10,7 +10,6 @@
 
 // Tests that an impl method's bounds aren't *more* restrictive
 // than the trait method it's implementing
-use core::iter::BaseIter;
 
 trait A {
   fn b<C:Copy,D>(x: C) -> C;

src/test/compile-fail/issue-2611-5.rs

@@ -10,7 +10,6 @@
 
 // Tests that ty params get matched correctly when comparing
 // an impl against a trait
-use core::iter::BaseIter;
 
 trait A {
   fn b<C:Copy,D>(x: C) -> C;

src/test/compile-fail/moves-based-on-type-cyclic-types-issue-4821.rs

@@ -13,8 +13,6 @@
 // See middle::ty::type_contents() for more information.
 
 extern mod std;
-use core::cmp::Ord;
-use core::option::swap_unwrap;
 
 struct List { key: int, next: Option<~List> }
 

src/test/compile-fail/regions-glb-free-free.rs

@@ -11,8 +11,6 @@
 mod argparse {
     extern mod std;
 
-    use core::either::{Either, Left, Right};
-
     pub struct Flag {
         name: &str,
         desc: &str,