- change all uses of Path in fn args to &P
- FileStream.read assumptions were wrong (libuv file io is non-positional)
- the above will mean that we "own" Seek impl info .. should probably
  push it in UvFileDescriptor..
- needs more tests

the test "touch"es a new file

This commit removes the "super_*" functions from
typeck::infer::combine, and adds them as default methods on the
Combine trait instead, making it possible to remove a lot of
boilerplate from the various impls of Combine.

I've been wanting to do this for over a year.  In fact, it was my
original motivation for default methods!

It might be possible to tighten things up even more, but this is the
bulk of it.

`stdtest` and `extratest` expects to be able to write to `tmp` directory under the current working directory, so the first commit creates `tmp` directory and changes the directory before running tests.

The second commit adds `--bench` argument to test runs and copies metrics from the remote device.

Change the type of crust fns like this one:

    extern fn foo() { ... }

from `*u8` to `extern "C" fn()`.

r? @pcwalton (or whomever)

Commit 0932ab33 accidentally broke rusti by making it parse the binary
name as the input, instead of parsing what the user typed.

Fixes #8615.

Fixes issue #8612.

Implement CharIterator as a separate struct, so that it can be .clone()'d. Fix `.char_range_at_reverse` so that it performs better, closer to the forwards version. This makes the reverse iterators and users like `.rfind()` perform better.

    Before
    test str::bench::char_iterator ... bench: 146 ns/iter (+/- 0)
    test str::bench::char_iterator_ascii ... bench: 397 ns/iter (+/- 49)
    test str::bench::char_iterator_rev ... bench: 576 ns/iter (+/- 8)
    test str::bench::char_offset_iterator ... bench: 128 ns/iter (+/- 2)
    test str::bench::char_offset_iterator_rev ... bench: 425 ns/iter (+/- 59)
    
    After
    test str::bench::char_iterator ... bench: 130 ns/iter (+/- 1)
    test str::bench::char_iterator_ascii ... bench: 307 ns/iter (+/- 5)
    test str::bench::char_iterator_rev ... bench: 185 ns/iter (+/- 8)
    test str::bench::char_offset_iterator ... bench: 131 ns/iter (+/- 13)
    test str::bench::char_offset_iterator_rev ... bench: 183 ns/iter (+/- 2)

To be able to use a string slice to represent the CharIterator, a function `slice_unchecked` is added, that does the same as `slice_bytes` but without any boundary checks.

It would be possible to implement CharIterator with pointer arithmetic to make it *much more efficient*, but since vec iterator is still improving, it's too early to attempt to re-implement it in other places. Hopefully CharIterator can be implemented on top of vec iterator without any unsafe code later.

Additional changes fix the documentation about null termination.

Nothing arguable here, as far as I can tell.

For #7083.

The metadata issue with the old version is now fixed. Ready for review.

This is also not the full solution to #7083, because this is not supported yet:
```
trait Foo : Send { }

impl <T: Send> Foo for T { }

fn foo<T: Foo>(val: T, chan: std::comm::Chan<T>) {
    chan.send(val);
}
```

cc @nikomatsakis