single buffer for exponent fmt of integers

[pascaldekloe]

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 #135265 and #136594).

r? libs

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[pascaldekloe]

https://rust.godbolt.org/z/oe9zr1qd9

Do you want to have another look @tgross35? 🙏 I think we can (and should) format floating points in a single buffer too. If that succeeds then we can drop unstable feature numfmt, package num::fmt. ✨

[tgross35]

Thanks for the ping, sure I'm happy to take the formatting followups here

r? tgross35