Auto merge of #68499 - ssomers:btree_search_tidying, r=Mark-Simulacrum

BtreeMap range_search spruced up

#39457 created a lower level entry point for `range_search` to operate on, but it's really not hard to move it up a level of abstraction, making it somewhat shorter and reusing existing unsafe code (`new_edge` is unsafe although it is currently not tagged as such).

Benchmark added. Comparison says there's no real difference:
```
>cargo benchcmp old3.txt new3.txt --threshold 5
 name                                           old3.txt ns/iter  new3.txt ns/iter  diff ns/iter   diff %  speedup
 btree::map::find_seq_100                       19                21                           2   10.53%   x 0.90
 btree::map::range_excluded_unbounded           3,117             2,838                     -279   -8.95%   x 1.10
 btree::map::range_included_unbounded           1,768             1,871                      103    5.83%   x 0.94
 btree::set::intersection_10k_neg_vs_10k_pos    35                37                           2    5.71%   x 0.95
 btree::set::intersection_staggered_100_vs_10k  2,488             2,314                     -174   -6.99%   x 1.08
 btree::set::is_subset_10k_vs_100               3                 2                           -1  -33.33%   x 1.50
```

r? @Mark-Simulacrum

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