Faster fmt::Display of 128-bit integers, without unsafe pointer

[pascaldekloe]

In followup of #135265, hereby the 128-bit part.

• Batches per 16 instead of 19 digits
• Buffer access as array insteaf of unsafe pointer
• Added test coverage for i128 and u128

r? tgross35 ChrisDenton

[pascaldekloe]

@bors try @rust-timer queue

[rust-timer]

Insufficient permissions to issue commands to rust-timer.

[bors]

@pascaldekloe: 🔑 Insufficient privileges: not in try users

[tgross35]

Do you have a link to the asm diff?

[bors]

☔ The latest upstream changes (presumably #136409) made this pull request unmergeable. Please resolve the merge conflicts.

[pascaldekloe]

Do you have a link to the asm diff?

https://rust.godbolt.org/z/6Pq3hKT8d

[pascaldekloe]

Constant calculation done with https://gist.github.com/pascaldekloe/df103c17ebf01920958053c76505aedf.

[tgross35]

I don't have any concerns here but the tests need to be fixed of course :)

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

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