Rollup merge of #111609 - LegionMammal978:internal-unsafe, r=thomcc

Mark internal functions and traits unsafe to reflect preconditions

No semantics are changed in this PR; I only mark some functions and and a trait `unsafe` which already had implicit preconditions. Although it seems somewhat redundant for `numfmt::Part::Copy` to contain a `&[u8]` instead of a `&str`, given that all of its current consumers ultimately expect valid UTF-8. Is the type also intended to work for byte-slice formatting in the future?

Author: Dylan-DPC