Rollup merge of #70111 - Mark-Simulacrum:btree-no-shared, r=cuviper

BTreeMap: remove shared root

This replaces the shared root with `Option`s in the BTreeMap code, and then slightly cleans up the node manipulation code taking advantage of the removal of the shared root. I expect that further simplification is possible, but wanted to get this posted for initial review.

Note that `BTreeMap::new()` continues to not allocate.

Benchmarks seem within the margin of error/unaffected, as expected for an entirely predictable branch.

```
 name                                 alloc-bench-a ns/iter  alloc-bench-b ns/iter  diff ns/iter  diff %  speedup
 btree::map::iter_mut_20              20                     21                                1   5.00%   x 0.95
 btree::set::clone_100                1,360                  1,439                            79   5.81%   x 0.95
 btree::set::clone_100_and_into_iter  1,319                  1,434                           115   8.72%   x 0.92
 btree::set::clone_10k                143,515                150,991                       7,476   5.21%   x 0.95
 btree::set::clone_10k_and_clear      142,792                152,916                      10,124   7.09%   x 0.93
 btree::set::clone_10k_and_into_iter  146,019                154,561                       8,542   5.85%   x 0.94
```

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