Expose whether event loops have active I/O

The green scheduler can optimize its runtime based on this by deciding to not go
to sleep in epoll() if there is no active I/O and there is a task to be stolen.

This is implemented for librustuv by keeping a count of the number of tasks
which are currently homed. If a task is homed, and then performs a blocking I/O
operation, the count will be nonzero while the task is blocked. The homing count
is intentionally 0 when there are I/O handles, but no handles currently blocked.
The reason for this is that epoll() would only be used to wake up the scheduler
anyway.

The crux of this change was to have a `HomingMissile` contain a mutable borrowed
reference back to the `HomeHandle`. The rest of the change was just dealing with
this fallout. This reference is used to decrement the homed handle count in a
HomingMissile's destructor.

Also note that the count maintained is not atomic because all of its
increments/decrements/reads are all on the same I/O thread.
Percolate the (Scheduler, GreenTask) pair upwards

This is in preparation for running do_work in a loop while there are no active
I/O handles. This changes the do_work and interpret_message_queue methods to
return a triple where the last element is a boolean flag as to whether work was
done or not.

This commit preserves the same behavior as before, it simply re-structures the
code in preparation for future work.
Don't hit epoll unless a scheduler absolutely must

Currently, a scheduler will hit epoll() or kqueue() at the end of *every task*.
The reason is that the scheduler will context switch back to the scheduler task,
terminate the previous task, and then return from run_sched_once. In doing so,
the scheduler will poll for any active I/O.

This shows up painfully in benchmarks that have no I/O at all. For example, this
benchmark:

    for _ in range(0, 1000000) {
        spawn(proc() {});
    }

In this benchmark, the scheduler is currently wasting a good chunk of its time
hitting epoll() when there's always active work to be done (run with
RUST_THREADS=1).

This patch uses the previous two commits to alter the scheduler's behavior to
only return from run_sched_once if no work could be found when trying really
really hard. If there is active I/O, this commit will perform the same as
before, falling back to epoll() to check for I/O completion (to not starve I/O
tasks).

In the benchmark above, I got the following numbers:

    12.554s on today's master
    3.861s  with #12172 applied
    2.261s  with both this and #12172 applied

cc #8341
mk: Fix non-android cross builds.
Removed num::Orderable
auto merge of #12061 : pongad/rust/delorderable, r=cmr

#12057
Fix a bug where cached stacks weren't re-used

The condition was the wrong direction and it also didn't take equality into
account. Tests were added for both cases.

For the small benchmark of `task::try(proc() {}).unwrap()`, this takes the
iteration time on OSX from 15119 ns/iter to 6179 ns/iter (timed with
RUST_THREADS=1)

cc #11389
Don't allocate in LocalHeap::new()

One of these is allocated for every task, trying to cut down on allocations

cc #11389
Don't require an allocation for on_exit messages

Instead, use an enum to allow running both a procedure and sending the task
result over a channel. I expect the common case to be sending on a channel (e.g.
task::try), so don't require an extra allocation in the common case.

cc #11389
Remove two allocations from spawning a green task

Two unfortunate allocations were wrapping a proc() in a proc() with
GreenTask::build_start_wrapper, and then boxing this proc in a ~proc() inside of
Context::new(). Both of these allocations were a direct result from two
conditions:

1. The Context::new() function has a nice api of taking a procedure argument to
   start up a new context with. This inherently required an allocation by
   build_start_wrapper because extra code needed to be run around the edges of a
   user-provided proc() for a new task.

2. The initial bootstrap code only understood how to pass one argument to the
   next function. By modifying the assembly and entry points to understand more
   than one argument, more information is passed through in registers instead of
   allocating a pointer-sized context.

This is sadly where I end up throwing mips under a bus because I have no idea
what's going on in the mips context switching code and don't know how to modify
it.

Closes #7767
cc #11389
auto merge of #12172 : alexcrichton/rust/green-improvements, r=brson

These commits pick off some low-hanging fruit which were slowing down spawning green threads. The major speedup comes from fixing a bug in stack caching where we never used any cached stacks!

The program I used to benchmark is at the end. It was compiled with `rustc --opt-level=3 bench.rs --test` and run as `RUST_THREADS=1 ./bench --bench`. I chose to use `RUST_THREADS=1` due to #11730 as the profiles I was getting interfered too much when all the schedulers were in play (and shouldn't be after #11730 is fixed). All of the units below are in ns/iter as reported by `--bench` (lower is better).

|               | green | native | raw    |
| ------------- | ----- | ------ | ------ |
| osx before    | 12699 | 24030  | 19734  |
| linux before  | 10223 | 125983 | 122647 |
| osx after     |  3847 | 25771  | 20835  |
| linux after   |  2631 | 135398 | 122765 |

Note that this is *not* a benchmark of spawning green tasks vs native tasks. I put in the native numbers just to get a ballpark of where green tasks are. This is benchmark is *clearly* benefiting from stack caching. Also, OSX is clearly not 5x faster than linux, I think my VM is just much slower.

All in all, this ended up being a nice 4x speedup for spawning a green task when you're using a cached stack.

```rust
extern mod extra;
extern mod native;
use std::rt::thread::Thread;

#[bench]
fn green(bh: &mut extra::test::BenchHarness) {
    let (p, c) = SharedChan::new();
    bh.iter(|| {
        let c = c.clone();
        spawn(proc() {
            c.send(());
        });
        p.recv();
    });
}

#[bench]
fn native(bh: &mut extra::test::BenchHarness) {
    let (p, c) = SharedChan::new();
    bh.iter(|| {
        let c = c.clone();
        native::task::spawn(proc() {
            c.send(());
        });
        p.recv();
    });
}

#[bench]
fn raw(bh: &mut extra::test::BenchHarness) {
    bh.iter(|| {
        Thread::start(proc() {}).join()
    });
}
```
auto merge of #12256 : brson/rust/android, r=alexcrichton

Android bot has been having some problems. A few details in the commits.
Tweak ItemDecorator API

The old method of building up a list of items and threading it through
all of the decorators was unwieldy and not really scalable as
non-deriving ItemDecorators become possible. The API is now that the
decorator gets an immutable reference to the item it's attached to, and
a callback that it can pass new items to. If we want to add syntax
extensions that can modify the item they're attached to, we can add that
later, but I think it'll have to be separate from ItemDecorator to avoid
strange ordering issues.
Refactored ast_map and friends, mainly to have Paths without storing them.
auto merge of #12162 : eddyb/rust/ast-map-cheap-path, r=nikomatsakis
auto merge of #12186 : alexcrichton/rust/no-sleep-2, r=brson

Any single-threaded task benchmark will spend a good chunk of time in `kqueue()` on osx and `epoll()` on linux, and the reason for this is that each time a task is terminated it will hit the syscall. When a task terminates, it context switches back to the scheduler thread, and the scheduler thread falls out of `run_sched_once` whenever it figures out that it did some work.

If we know that `epoll()` will return nothing, then we can continue to do work locally (only while there's work to be done). We must fall back to `epoll()` whenever there's active I/O in order to check whether it's ready or not, but without that (which is largely the case in benchmarks), we can prevent the costly syscall and can get a nice speedup.

I've separated the commits into preparation for this change and then the change itself, the last commit message has more details.
auto merge of #12192 : luqmana/rust/fix-cross, r=alexcrichton

Fix some fall out from the big command line option changes.
auto merge of #12234 : sfackler/rust/restructure-item-decorator, r=huonw

The old method of building up a list of items and threading it through
all of the decorators was unwieldy and not really scalable as
non-deriving ItemDecorators become possible. The API is now that the
decorator gets an immutable reference to the item it's attached to, and
a callback that it can pass new items to. If we want to add syntax
extensions that can modify the item they're attached to, we can add that
later, but I think it'll have to be separate from ItemDecorator to avoid
strange ordering issues.

@huonw
Upgrade LLVM

Includes an upstream commit by pcwalton to improve codegen of our enums getting
moved around.
auto merge of #12207 : alexcrichton/rust/up-llvm, r=sfackler

Includes an upstream commit by pcwalton to improve codegen of our enums getting
moved around.

This also introduces a new commit on top of our stack of patches to fix a mingw32 build issue. I have submitted the patch upstream: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140210/204653.html

I verified that this builds on the try bots, which amazes me because I think that c++11 is turned on now, but I guess we're still lucky!

Closes #10613 (pcwalton's patch landed)
Closes #11992 (llvm has removed these options)