Group unused import warnings per path list

Given a file

```rust
use std::collections::{BinaryHeap, BTreeMap, BTreeSet};

fn main() {}
```

Show a single warning, instead of three for each unused import:

```nocode
warning: unused imports, #[warn(unused_imports)] on by default
 --> foo.rs:1:24
  |
1 | use std::collections::{BinaryHeap, BTreeMap, BTreeSet};
  |                        ^^^^^^^^^^  ^^^^^^^^  ^^^^^^^^
```

Include support for lints pointing at `MultilineSpan`s, instead of just
`Span`s.

src/test/compile-fail/lint-unused-imports.rs

@@ -17,8 +17,9 @@ use std::mem::*;            // shouldn't get errors for not using
                             // everything imported
 
 // Should get errors for both 'Some' and 'None'
-use std::option::Option::{Some, None}; //~ ERROR unused import: `Some`
-                                    //~^ ERROR unused import: `None`
+use std::option::Option::{Some, None};
+//~^ ERROR unused imports: `None`, `Some`
+//~| ERROR unused imports: `None`, `Some`
 
 use test::A;       //~ ERROR unused import: `test::A`
 // Be sure that if we just bring some methods into scope that they're also

src/test/ui/span/multispan-import-lint.rs

@@ -0,0 +1,15 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::cmp::{Eq, Ord, min, PartialEq, PartialOrd};
+
+fn main() {
+    let _ = min(1, 2);
+}

src/test/ui/span/multispan-import-lint.stderr

@@ -0,0 +1,6 @@
+warning: unused imports: `Eq`, `Ord`, `PartialEq`, `PartialOrd`, #[warn(unused_imports)] on by default
+  --> $DIR/multispan-import-lint.rs:11:16
+   |
+11 | use std::cmp::{Eq, Ord, min, PartialEq, PartialOrd};
+   |                ^^  ^^^       ^^^^^^^^^  ^^^^^^^^^^
+