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: