Decouple integer formatting from std::num::strconv

This works towards a complete rewrite and ultimate removal of the `std::num::strconv` module (see #6220), and the removal of the `ToStrRadix` trait in favour of using the `std::fmt` functionality directly. This should make for a cleaner API, encourage less allocation, and make the implementation far more comprehensible.

The `Formatter::pad_integral` method has also been refactored make it easier to understand.

The formatting tests for integers have been moved out of `run-pass/ifmt.rs` in order to provide more immediate feedback when building using `make check-stage2-std NO_REBUILD=1`.

The benchmarks have been standardised between std::num::strconv and std::num::fmt to make it easier to compare the performance of the different implementations.

Arbitrary radixes are now easier to use in format strings. For example:

~~~
assert_eq!(format!("{:04}", radix(3, 2)), ~"0011");
~~~

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