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()
    });
}
```
Refactored ast_map and friends, mainly to have Paths without storing them.