Auto merge of #135265 - pascaldekloe:fmt-int-speed, r=tgross35,ChrisDenton

Display of integers without raw pointers and without overflowing_literals

The benchmarks as is measure formatting speed of literals. The first commit `black_box`-es input to simulate runtime speed instead.

The second commit replaces `unsafe` pointer optimizations with plain array indices. The performance is equivalent on Apple M1. Needs peer review on Intel.

Happy to do the 128-bit version too if such change is welcome.

Author: bors