syntax::ext: replace span_fatal with span_err in many places.

This means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.
Make WindowsPath::new("C:foo").root_path() return Some("C:")
Added benchmark for pow and pow_with_uint
Replace old pow_with_uint with the new pow func

There was an old and barely used implementation of pow, which expected
both parameters to be uint and required more traits to be implemented.
Since a new implementation for `pow` landed, I'm proposing to remove
this old impl in favor of the new one.

The benchmark shows that the new implementation is faster than the one
being removed:

test num::bench::bench_pow_function               ..bench:      9429 ns/iter (+/- 2055)
test num::bench::bench_pow_with_uint_function     ...bench:     28476 ns/iter (+/- 2202)
Squashed commit of the following:

commit d00623d60afd460755b749ad5f94935f756f29d2
Author: musitdev <philippe.delrieu@free.fr>
Date:   Sat Jan 4 22:31:40 2014 +0100

    correct last comments.

commit ef09d6b6d1eebbd7c713c9dad96ed7bfc19dd884
Author: musitdev <philippe.delrieu@free.fr>
Date:   Thu Jan 2 20:28:53 2014 +0100

    update with the last remarks.

commit 46a028fe1fcdc2a7dcdd78a63001793eff614349
Author: musitdev <philippe.delrieu@free.fr>
Date:   Thu Jan 2 10:17:18 2014 +0100

    wrap example code in main function.

commit 2472901929bef09786b7aef8ca7c89fbe67d8e3e
Author: musitdev <philippe.delrieu@free.fr>
Date:   Mon Dec 30 19:32:46 2013 +0100

    Correct code to compile.

commit ed96b2223176781743e984af0e19abcb82150f1f
Author: musitdev <philippe.delrieu@free.fr>
Date:   Thu Dec 5 11:32:28 2013 +0100

    Correct the comment based on the PR comment.
    Change init call to new to reflect last change.

commit 38b0390c3533a16f822a6df5f90b907bd8ed6edc
Author: musitdev <philippe.delrieu@free.fr>
Date:   Wed Dec 4 22:34:25 2013 +0100

    correct from_utf8_owned call.

commit 08bed4c5f4fadf93ec457b605a1a1354323d2f5c
Author: musitdev <philippe.delrieu@free.fr>
Date:   Wed Dec 4 22:12:41 2013 +0100

    correct code '''

commit 02fddcbe2ab37fe842872691105bc4c5cff5abb5
Author: musitdev <philippe.delrieu@free.fr>
Date:   Wed Dec 4 13:25:54 2013 +0100

    correct typing error

commit b26830b8ddb49f551699e791832ed20640a0fafc
Author: musitdev <philippe.delrieu@free.fr>
Date:   Wed Dec 4 13:18:39 2013 +0100

    pass make check

commit e87c4f53286122efd0d2364ea45600d4fa4d5744
Author: musitdev <philippe.delrieu@free.fr>
Date:   Wed Dec 4 10:47:24 2013 +0100

    Add Json example and documentation.
extra::json: add documentation and examples
extra::json: add documentation and examples
auto merge of #11643 : kballard/rust/path-root-path, r=erickt
auto merge of #11644 : huonw/rust/less-fatality, r=cmr

This means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.
auto merge of #10801 : musitdev/rust/jsondoc2, r=cmr

I update the example of json use to the last update of the json.rs code. I delete the old branch.
From my last request, I remove the example3 because it doesn't compile. I don't understand why and I don't have the time now to investigate.
auto merge of #11649 : FlaPer87/rust/pow, r=cmr

There was an old and barely used implementation of pow, which expected
both parameters to be uint and required more traits to be implemented.
Since a new implementation for `pow` landed, I'm proposing to remove
this old impl in favor of the new one.

The benchmark shows that the new implementation is faster than the one being removed:

```
    test num::bench::bench_pow_function               ..bench:      9429 ns/iter (+/- 2055)
    test num::bench::bench_pow_with_uint_function     ...bench:     28476 ns/iter (+/- 2202)
```