Auto merge of #136594 - pascaldekloe:fmt-int128, r=tgross35

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

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

Author: bors