Closes rust-lang#14210 (Make Vec.truncate() resilient against failure in Drop)
Closes rust-lang#14206 (Register new snapshots)
Closes rust-lang#14205 (use sched_yield on linux and freebsd)
Closes rust-lang#14204 (Add a crate for missing stubs from libcore)
Closes rust-lang#14201 (Render not_found with an absolute path to the rust stylesheet)
Closes rust-lang#14198 (update valgrind headers)
Closes rust-lang#14174 (Optimize common path of Once::doit)
Closes rust-lang#14162 (Print 'rustc' and 'rustdoc' as the command name for --version)
Closes rust-lang#14145 (Better strict version hash (SVH) computation)

Submitting PR again, because I cannot reopen #13349, and github does not attach new patch to that PR. 

=======

Optimize `Once::doit`: perform optimistic check that initializtion is
already completed.  `load` is much cheaper than `fetch_add` at least
on x86_64.

Verified with this test:

```
static mut o: one::Once = one::ONCE_INIT;
unsafe {
    loop {
        let start = time::precise_time_ns();
        let iters = 50000000u64;
        for _ in range(0, iters) {
            o.doit(|| { println!("once!"); });
        }
        let end = time::precise_time_ns();
        let ps_per_iter = 1000 * (end - start) / iters;
        println!("{} ps per iter", ps_per_iter);

        // confuse the optimizer
        o.doit(|| { println!("once!"); });
    }
}
```

Test executed on Mac, Intel Core i7 2GHz. Result is:
* 20ns per iteration without patch
*  4ns per iteration with this patch applied

Once.doit could be even faster (800ps per iteration), if `doit` function
was split into a pair of `doit`/`doit_slow`, and `doit` marked as
`#[inline]` like this:

```
#[inline(always)]
pub fn doit(&self, f: ||) {
    if self.cnt.load(atomics::SeqCst) < 0 {
        return
    }

    self.doit_slow(f);
}

fn doit_slow(&self, f: ||) { ... }
```

Use sync::one::Once to fetch the mach_timebase_info only once when
running precise_time_ns(). This helps because mach_timebase_info() is
surprisingly inefficient. Also fix the order of operations when applying
the timebase to the mach absolute time value.

This improves the time on my machine from

```
test tests::bench_precise_time_ns ... bench:       157 ns/iter (+/- 4)
```

to

```
test tests::bench_precise_time_ns ... bench:        38 ns/iter (+/- 3)
```

and it will get even faster once rust-lang#14174 lands.

…chton

Use sync::one::Once to fetch the mach_timebase_info only once when
running precise_time_ns(). This helps because mach_timebase_info() is
surprisingly inefficient. Also fix the order of operations when applying
the timebase to the mach absolute time value.

This improves the time on my machine from

```
test tests::bench_precise_time_ns ... bench:       157 ns/iter (+/- 4)
```

to

```
test tests::bench_precise_time_ns ... bench:        38 ns/iter (+/- 3)
```

and it will get even faster once #14174 lands.

… `.clone()` (rust-lang#14174)

fixes rust-lang#12357

changelog: [`useless_asref`]: don't suggest to use `.clone()` if the
target type doesn't implement the `Clone` trait