Rewrite integer formatting

This is PR is the beginning of 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 more comprehensible .

The `Formatter::{pad_integral, with_padding}` methods have also been refactored make things 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`.

Arbitrary radixes are now easier to use in format strings. For example:

~~~rust
assert_eq!(format!("{:04}", radix(3, 2)), ~"0011");
~~~

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.

~~~
 type | radix | std::num::strconv      | std::num::fmt
======|=======|========================|======================
 int  | bin   | 1748 ns/iter (+/- 150) | 321 ns/iter (+/- 25)
 int  | oct   |  706 ns/iter (+/- 53)  | 179 ns/iter (+/- 22)
 int  | dec   |  640 ns/iter (+/- 59)  | 207 ns/iter (+/- 10)
 int  | hex   |  637 ns/iter (+/- 77)  | 205 ns/iter (+/- 19)
 int  | 36    |  446 ns/iter (+/- 30)  | 309 ns/iter (+/- 20)
------|-------|------------------------|----------------------
 uint | bin   | 1724 ns/iter (+/- 159) | 322 ns/iter (+/- 13)
 uint | oct   |  663 ns/iter (+/- 25)  | 175 ns/iter (+/- 7)
 uint | dec   |  613 ns/iter (+/- 30)  | 186 ns/iter (+/- 6)
 uint | hex   |  519 ns/iter (+/- 44)  | 207 ns/iter (+/- 20)
 uint | 36    |  418 ns/iter (+/- 16)  | 308 ns/iter (+/- 32)
~~~