Rollup merge of rust-lang#145940 - pascaldekloe:int-exp-tune, r=tgross35

single buffer for exponent fmt of integers

No need for fragmented buffers when formatting.

```
orig.txt: fmt::write_i128_exp                                                  143.39ns/iter      +/- 0.32
orig.txt: fmt::write_i64_exp                                                    68.72ns/iter      +/- 0.03
new.txt:  fmt::write_i128_exp                                                  138.29ns/iter      +/- 0.50
new.txt:  fmt::write_i64_exp                                                    58.93ns/iter      +/- 4.62
```

This patch fully eliminates unsafe pointer use (after rust-lang#135265 and rust-lang#136594).

    r? libs

Author: tgross35