Auto merge of #67073 - ssomers:btree_navigation, r=cuviper

Bundle and document 6 BTreeMap navigation algorithms

- Expose a function to step through trees, without necessarily extracting the KV pair, that helps future operations like drain/retain (as demonstrated in [this drain_filter implementation](https://github.com/ssomers/rust/compare/btree_navigation_v3...ssomers:btree_drain_filter?expand=1))
- ~~Also aligns the implementation of the 2 x 3 iterators already using such navigation:~~
  - ~~Delay the moment the K,V references are plucked from the tree, for the 4 iterators on immutable and owned maps, just for symmetry. The same had to be done to the two iterators for mutable maps in #58431.~~
  - ~~Always explicitly use ptr::read to derive two handles from one handle. While the existing implementations for immutable maps (i.e. Range::next_unchecked and Range::next_back_unchecked) rely on implicit copying. There's no change in unsafe tags because these two functions were already (erroneously? prophetically?) tagged unsafe. I don't know whether they should be tagged unsafe. I guess they should be for mutable and owned maps, because you can change the map through one handle and leave the other handle invalid.~~
  - Preserve the way two handles are (temporarily) derived from one handle: implementations for immutable maps (i.e. Range::next_unchecked and Range::next_back_unchecked) rely on copying (implicitly before, explicitly now) and the others do `ptr::read`.
  - ~~I think the functions that support iterators on immutable trees (i.e. `Range::next_unchecked` and `Range::next_back_unchecked`) are erroneously tagged unsafe since you can already create multiple instances of such ranges, thus obtain multiple handles into the same tree. I did not change that but removed unsafe from the functions underneath.~~

Tested with miri in liballoc/tests/btree, except those that should_panic.

cargo benchcmp the best of 3 samples of all btree benchmarks before and after this PR:
```
name                                           old1.txt ns/iter  new2.txt ns/iter  diff ns/iter  diff %  speedup
btree::map::find_rand_100                      17                17                           0   0.00%   x 1.00
btree::map::find_rand_10_000                   57                55                          -2  -3.51%   x 1.04
btree::map::find_seq_100                       17                17                           0   0.00%   x 1.00
btree::map::find_seq_10_000                    42                39                          -3  -7.14%   x 1.08
btree::map::first_and_last_0                   14                14                           0   0.00%   x 1.00
btree::map::first_and_last_100                 36                37                           1   2.78%   x 0.97
btree::map::first_and_last_10k                 52                52                           0   0.00%   x 1.00
btree::map::insert_rand_100                    34                34                           0   0.00%   x 1.00
btree::map::insert_rand_10_000                 34                34                           0   0.00%   x 1.00
btree::map::insert_seq_100                     46                46                           0   0.00%   x 1.00
btree::map::insert_seq_10_000                  90                89                          -1  -1.11%   x 1.01
btree::map::iter_1000                          2,811             2,771                      -40  -1.42%   x 1.01
btree::map::iter_100000                        360,635           355,995                 -4,640  -1.29%   x 1.01
btree::map::iter_20                            39                42                           3   7.69%   x 0.93
btree::map::iter_mut_1000                      2,662             2,864                      202   7.59%   x 0.93
btree::map::iter_mut_100000                    336,825           346,550                  9,725   2.89%   x 0.97
btree::map::iter_mut_20                        40                43                           3   7.50%   x 0.93
btree::set::build_and_clear                    4,184             3,994                     -190  -4.54%   x 1.05
btree::set::build_and_drop                     4,151             3,976                     -175  -4.22%   x 1.04
btree::set::build_and_into_iter                4,196             4,155                      -41  -0.98%   x 1.01
btree::set::build_and_pop_all                  5,176             5,241                       65   1.26%   x 0.99
btree::set::build_and_remove_all               6,868             6,903                       35   0.51%   x 0.99
btree::set::difference_random_100_vs_100       721               691                        -30  -4.16%   x 1.04
btree::set::difference_random_100_vs_10k       2,420             2,411                       -9  -0.37%   x 1.00
btree::set::difference_random_10k_vs_100       54,726            52,215                  -2,511  -4.59%   x 1.05
btree::set::difference_random_10k_vs_10k       164,384           170,256                  5,872   3.57%   x 0.97
btree::set::difference_staggered_100_vs_100    739               709                        -30  -4.06%   x 1.04
btree::set::difference_staggered_100_vs_10k    2,320             2,265                      -55  -2.37%   x 1.02
btree::set::difference_staggered_10k_vs_10k    68,020            70,246                   2,226   3.27%   x 0.97
btree::set::intersection_100_neg_vs_100_pos    23                24                           1   4.35%   x 0.96
btree::set::intersection_100_neg_vs_10k_pos    28                29                           1   3.57%   x 0.97
btree::set::intersection_100_pos_vs_100_neg    24                24                           0   0.00%   x 1.00
btree::set::intersection_100_pos_vs_10k_neg    28                28                           0   0.00%   x 1.00
btree::set::intersection_10k_neg_vs_100_pos    27                27                           0   0.00%   x 1.00
btree::set::intersection_10k_neg_vs_10k_pos    30                29                          -1  -3.33%   x 1.03
btree::set::intersection_10k_pos_vs_100_neg    27                28                           1   3.70%   x 0.96
btree::set::intersection_10k_pos_vs_10k_neg    29                29                           0   0.00%   x 1.00
btree::set::intersection_random_100_vs_100     592               572                        -20  -3.38%   x 1.03
btree::set::intersection_random_100_vs_10k     2,271             2,269                       -2  -0.09%   x 1.00
btree::set::intersection_random_10k_vs_100     2,301             2,333                       32   1.39%   x 0.99
btree::set::intersection_random_10k_vs_10k     147,879           150,148                  2,269   1.53%   x 0.98
btree::set::intersection_staggered_100_vs_100  622               632                         10   1.61%   x 0.98
btree::set::intersection_staggered_100_vs_10k  2,101             2,032                      -69  -3.28%   x 1.03
btree::set::intersection_staggered_10k_vs_10k  60,341            61,834                   1,493   2.47%   x 0.98
btree::set::is_subset_100_vs_100               417               426                          9   2.16%   x 0.98
btree::set::is_subset_100_vs_10k               1,281             1,324                       43   3.36%   x 0.97
btree::set::is_subset_10k_vs_100               2                 2                            0   0.00%   x 1.00
btree::set::is_subset_10k_vs_10k               41,054            41,612                     558   1.36%   x 0.99
```

r? cuviper