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.

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%
```

0 files changed, 0 insertions, 0 deletions