Safer sort partition

[kornelski]

This reduces amount of unsafe code in partition(), while generating essentially the same code.

partition() and functions calling it can only run into out-of-bounds issues if the is_less function is buggy, so I've used cold panic_on_ord_violation() to replace various other panics/aborts. This slightly reduces code size, and gives a more relevant error message.

I've verified that the intrinsics::abort() was unnecessary: 13d7b54#r162627329

Related to #144327

[rustbot]

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