Auto merge of #142294 - GuillaumeGomez:specialize-tostring-on-128-integers, r=tgross35

Use a distinct `ToString` implementation for `u128` and `i128`

Part of rust-lang/rust#135543.

Follow-up of rust-lang/rust#136264.

When working on rust-lang/rust#142098, I realized that `i128` and `u128` could also benefit from a distinct `ToString` implementation so here it.

The last commit is just me realizing that I forgot to add the format tests for `usize` and `isize`.

Here is the bench comparison:

| bench name | last nightly | with this PR | diff |
|-|-|-|-|
| bench_i128 | 29.25 ns/iter (+/- 0.66) | 17.52 ns/iter (+/- 0.7) | -40.1% |
| bench_u128 | 34.06 ns/iter (+/- 0.21) | 16.1 ns/iter (+/- 0.6) | -52.7% |

I used this code to test:

```rust
#![feature(test)]

extern crate test;

use test::{Bencher, black_box};

#[inline(always)]
fn convert_to_string<T: ToString>(n: T) -> String {
    n.to_string()
}

macro_rules! decl_benches {
    ($($name:ident: $ty:ident,)+) => {
        $(
	    #[bench]
            fn $name(c: &mut Bencher) {
                c.iter(|| convert_to_string(black_box({ let nb: $ty = 20; nb })));
            }
	)+
    }
}

decl_benches! {
    bench_u128: u128,
    bench_i128: i128,
}
```

Author: bors