Add dead-code warning pass

PR for issue #1749 mainly to get some feedback and suggestion. This adds a pass that warns if a function, struct, enum, or static item is never used. For the following code,

```rust
pub static pub_static: int = 0;
static priv_static: int = 0;
static used_static: int = 0;

pub fn pub_fn() { used_fn(); }
fn priv_fn() { let unused_struct = PrivStruct; }
fn used_fn() {}

pub struct PubStruct();
struct PrivStruct();
struct UsedStruct1 { x: int }
struct UsedStruct2(int);
struct UsedStruct3();

pub enum pub_enum { foo1, bar1 }
enum priv_enum { foo2, bar2 }
enum used_enum { foo3, bar3 }

fn foo() {
	bar();
	let unused_enum = foo2;
}

fn bar() {
	foo();
}

fn main() {
	let used_struct1 = UsedStruct1 { x: 1 };
	let used_struct2 = UsedStruct2(1);
	let used_struct3 = UsedStruct3;
	let t = used_static;
	let e = foo3;
}
```

it would add the following warnings:

```rust
/home/ktt3ja/test.rs:2:0: 2:28 warning: code is never used: `priv_static`, #[warn(dead_code)] on by default
/home/ktt3ja/test.rs:2 static priv_static: int = 0;
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/ktt3ja/test.rs:6:0: 6:48 warning: code is never used: `priv_fn`, #[warn(dead_code)] on by default
/home/ktt3ja/test.rs:6 fn priv_fn() { let unused_struct = PrivStruct; }
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/ktt3ja/test.rs:10:0: 10:20 warning: code is never used: `PrivStruct`, #[warn(dead_code)] on by default
/home/ktt3ja/test.rs:10 struct PrivStruct();
                        ^~~~~~~~~~~~~~~~~~~~
/home/ktt3ja/test.rs:16:0: 16:29 warning: code is never used: `priv_enum`, #[warn(dead_code)] on by default
/home/ktt3ja/test.rs:16 enum priv_enum { foo2, bar2 }
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/ktt3ja/test.rs:19:0: 22:1 warning: code is never used: `foo`, #[warn(dead_code)] on by default
/home/ktt3ja/test.rs:19 fn foo() {
/home/ktt3ja/test.rs:20 	bar();
/home/ktt3ja/test.rs:21 	let unused_enum = foo2;
/home/ktt3ja/test.rs:22 }
/home/ktt3ja/test.rs:24:0: 26:1 warning: code is never used: `bar`, #[warn(dead_code)] on by default
/home/ktt3ja/test.rs:24 fn bar() {
/home/ktt3ja/test.rs:25 	foo();
/home/ktt3ja/test.rs:26 }
```

Furthermore, I would like to solicit some test cases since I haven't tested extensively and I'm still unclear about some of the things in here. For example, I'm not sure how reexports would affect this and just assumed that LiveContext (which is a copy of reachable::ReachableContext) does enough work to handle it. Also, the test case above doesn't include any impl or methods, etc.