Rollup merge of #39538 - stjepang:slightly-optimize-sort, r=alexcrichton
Slightly optimize slice::sort
First, get rid of some bound checks.
Second, instead of comparing by ternary `compare` function, use a binary function testing whether an element is less than some other element. This apparently makes it easier for the compiler to reason about the code. I've noticed the same effect with [pdqsort](https://github.com/stjepang/pdqsort) crate.
Benchmark:
```
name before ns/iter after ns/iter diff ns/iter diff %
slice::bench::sort_large_ascending 8,969 (8919 MB/s) 7,410 (10796 MB/s) -1,559 -17.38%
slice::bench::sort_large_big_ascending 355,640 (3599 MB/s) 359,137 (3564 MB/s) 3,497 0.98%
slice::bench::sort_large_big_descending 427,112 (2996 MB/s) 424,721 (3013 MB/s) -2,391 -0.56%
slice::bench::sort_large_big_random 2,207,799 (579 MB/s) 2,138,804 (598 MB/s) -68,995 -3.13%
slice::bench::sort_large_descending 13,694 (5841 MB/s) 13,514 (5919 MB/s) -180 -1.31%
slice::bench::sort_large_mostly_ascending 239,697 (333 MB/s) 203,542 (393 MB/s) -36,155 -15.08%
slice::bench::sort_large_mostly_descending 270,102 (296 MB/s) 234,263 (341 MB/s) -35,839 -13.27%
slice::bench::sort_large_random 513,406 (155 MB/s) 470,084 (170 MB/s) -43,322 -8.44%
slice::bench::sort_large_random_expensive 23,650,321 (3 MB/s) 23,675,098 (3 MB/s) 24,777 0.10%
slice::bench::sort_medium_ascending 143 (5594 MB/s) 132 (6060 MB/s) -11 -7.69%
slice::bench::sort_medium_descending 197 (4060 MB/s) 188 (4255 MB/s) -9 -4.57%
slice::bench::sort_medium_random 3,358 (238 MB/s) 3,271 (244 MB/s) -87 -2.59%
slice::bench::sort_small_ascending 32 (2500 MB/s) 32 (2500 MB/s) 0 0.00%
slice::bench::sort_small_big_ascending 97 (13195 MB/s) 97 (13195 MB/s) 0 0.00%
slice::bench::sort_small_big_descending 247 (5182 MB/s) 249 (5140 MB/s) 2 0.81%
slice::bench::sort_small_big_random 502 (2549 MB/s) 498 (2570 MB/s) -4 -0.80%
slice::bench::sort_small_descending 55 (1454 MB/s) 61 (1311 MB/s) 6 10.91%
slice::bench::sort_small_random 358 (223 MB/s) 356 (224 MB/s) -2 -0.56%
```
a419dd1c01