std::rand: move normal and exponential to their own file.
std::rand: implement the log-normal distribution.
Register new snapshots

This transitions the snapshot dependency process to understand that our
snapshots are now a single static binary rather than an array of files.
Add dead-code warning pass
Remove dead codes
std::rand: implement the chi-squared distribution.
std::rand: implement the F distribution.
std::rand: implement the student t distribution.
auto merge of #10855 : alexcrichton/rust/snapshots, r=pcwalton

This transitions the snapshot dependency process to understand that our
snapshots are now a single static binary rather than an array of files.
auto merge of #10477 : ktt3ja/rust/dead-code, r=alexcrichton

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.
encode trait lifetime params in metadata to allow cross-crate usage
auto merge of #10813 : dwrensha/rust/xcrate-lifetime-param, r=huonw

Before applying this patch, the included testcase fails with:
```
src/test/run-pass/xcrate-trait-lifetime-param.rs:20:10: 20:28 error: wrong number of lifetime parameters: expected 0 but found 1
src/test/run-pass/xcrate-trait-lifetime-param.rs:20 impl <'a> other::FromBuf<'a> for Reader<'a> {
                                                              ^~~~~~~~~~~~~~~~~~
```

There's another example in my comments to #10506.
Fix comment on ast::DefStruct
auto merge of #10866 : ktt3ja/rust/edit-doc, r=huonw

A comment that I previously added to ast::DefStruct is incorrect. Here's the modification.
Accept unsugared docs in missing-doc lint

Closes #10853
Use LLVM integrated assembler on Windows too.
Disable failing test.
auto merge of #10867 : sfackler/rust/unsugared-doc, r=huonw

Closes #10853
auto merge of #10874 : vadimcn/rust/integrated-as, r=alexcrichton

Last LLVM update seems to have fixed whatever prevented LLVM integrated assembler from generating correct unwind tables on Windows.   This PR switches Windows builds to use internal assembler by default.
Compilation via external assembler can still be requested via the newly added `-Z no-integrated-as` option.

Closes #8809
auto merge of #10859 : huonw/rust/helper-dists, r=cmr

This moves `std::rand::distribitions::{Normal, StandardNormal}` to `...::distributions::normal`, reexporting `Normal` from `distributions` (and similarly for `Exp` and Exp1`), and adds:
- Log-normal
- Chi-squared
- F
- Student T

all of which are implemented in C++11's random library. Tests in https://github.com/huonw/random-tests/commit/0424b8aded5e608ae386c1f917934a726d9cac6a. Note that these are approximately half documentation & half implementation (of which a significant portion is boilerplate `}`'s and so on).