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.
compiletest: Run all android tests serially

This is an attempt to isolate test failures on the bots. It may also
eliminate problems with the emulators breaking by reducing the chance
of OOM.
Stop looping on error waiting for android test results

These seem to be causing iloops on the bots. Let's rather see the errors.
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)
return value/use extra::test::black_box in benchmarks
extra: Capture stdout/stderr of tests by default

When tests fail, their stdout and stderr is printed as part of the summary, but
this helps suppress failure messages from #[should_fail] tests and generally
clean up the output of the test runner.
Don't copy &Trait and &mut Trait to temporaries for every call.
Ensure an error is raised on infinite recursion
Add function doc comments for extra::url::*