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)
```

src/libextra/json.rs

@@ -909,7 +909,7 @@ impl<T : Iterator<char>> Parser<T> {
             }
         }
 
-        let exp: f64 = num::pow_with_uint(10u, exp);
+        let exp: f64 = num::pow(10u as f64, exp);
         if neg_exp {
             res /= exp;
         } else {