Auto merge of #136264 - GuillaumeGomez:optimize-integers-to-string, r=Amanieu

Optimize `ToString` implementation for integers

Part of rust-lang/rust#135543.

Follow-up of rust-lang/rust#133247 and rust-lang/rust#128204.

The benchmark results are:

| name| 1.87.0-nightly (3ea711f17 2025-03-09) | With this PR | diff |
|-|-|-|-|
| bench_i16 | 32.06 ns/iter (+/- 0.12) | 17.62 ns/iter (+/- 0.03) | -45% |
| bench_i32 | 31.61 ns/iter (+/- 0.04) | 15.10 ns/iter (+/- 0.06) | -52% |
| bench_i64 | 31.71 ns/iter (+/- 0.07) | 15.02 ns/iter (+/- 0.20) | -52% |
| bench_i8 | 13.21 ns/iter (+/- 0.14) | 14.93 ns/iter (+/- 0.16) | +13% |
| bench_u16 | 31.20 ns/iter (+/- 0.06) | 16.14 ns/iter (+/- 0.11) | -48% |
| bench_u32 | 33.27 ns/iter (+/- 0.05) | 16.18 ns/iter (+/- 0.10) | -51% |
| bench_u64 | 31.44 ns/iter (+/- 0.06) | 16.62 ns/iter (+/- 0.21) | -47% |
| bench_u8 | 10.57 ns/iter (+/- 0.30) | 13.00 ns/iter (+/- 0.43) | +22% |

More information about it in [the original comment](rust-lang/rust#136264 (comment)).

r? `@workingjubilee`

Author: bors