BTreeMap: let NodeRef point to the actual type of the node

[ssomers]

Building on #78104, it's relatively easy to confine mangled target types to the space-efficient BoxedNode, and let both the map's roots and node references on the fly point to the correct type. This reduces casts to two places: one for unpacking a boxed node to its actual target, and one for Global::dealloc.

[rust-highfive]

r? @withoutboats

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[ssomers]

r? @Mark-Simulacrum

[ssomers]

Performance is affected:

 name                                           old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::clone_fat_val_100_and_remove_all   82,461       69,990            -12,471  -15.12%   x 1.18
 btree::map::clone_slim_100                     2,609        2,096                -513  -19.66%   x 1.24
 btree::map::clone_slim_100_and_clear           2,598        2,112                -486  -18.71%   x 1.23
 btree::map::clone_slim_100_and_into_iter       2,587        2,103                -484  -18.71%   x 1.23
 btree::map::clone_slim_100_and_remove_half     3,694        3,314                -380  -10.29%   x 1.11
 btree::map::clone_slim_10k_and_drain_half      332,653      374,100            41,447   12.46%   x 0.89
 btree::map::clone_slim_10k_and_remove_all      575,600      643,555            67,955   11.81%   x 0.89
 btree::map::first_and_last_0                   10           34                     24  240.00%   x 0.29
 btree::map::first_and_last_100                 35           45                     10   28.57%   x 0.78
 btree::map::first_and_last_10k                 57           73                     16   28.07%   x 0.78
 btree::map::iter_100                           12,039       10,812             -1,227  -10.19%   x 1.11
 btree::map::iteration_mut_1000                 3,664        3,127                -537  -14.66%   x 1.17
 btree::map::iteration_mut_100000               518,260      436,225           -82,035  -15.83%   x 1.19
 btree::map::iteration_mut_20                   66           56                    -10  -15.15%   x 1.18
 btree::map::range_included_excluded            416,977      461,305            44,328   10.63%   x 0.90
 btree::map::range_included_unbounded           144,098      177,614            33,516   23.26%   x 0.81
 btree::map::range_unbounded_unbounded          77,250       104,874            27,624   35.76%   x 0.74
 btree::set::clone_100_and_drain_half           2,366        2,887                 521   22.02%   x 0.82
 btree::set::clone_100_and_pop_all              2,934        3,234                 300   10.22%   x 0.91
 btree::set::clone_100_and_remove_all           3,370        4,887               1,517   45.01%   x 0.69
 btree::set::clone_100_and_remove_half          2,368        2,946                 578   24.41%   x 0.80
 btree::set::clone_10k_and_drain_half           272,063      319,056            46,993   17.27%   x 0.85
 btree::set::clone_10k_and_remove_all           394,995      549,870           154,875   39.21%   x 0.72
 btree::set::clone_10k_and_remove_half          423,185      495,710            72,525   17.14%   x 0.85
 btree::set::difference_random_100_vs_100       707          859                   152   21.50%   x 0.82
 btree::set::difference_random_100_vs_10k       2,363        2,790                 427   18.07%   x 0.85
 btree::set::difference_random_10k_vs_100       52,948       63,523             10,575   19.97%   x 0.83
 btree::set::difference_staggered_100_vs_100    682          917                   235   34.46%   x 0.74
 btree::set::difference_staggered_100_vs_10k    2,200        2,722                 522   23.73%   x 0.81
 btree::set::difference_staggered_10k_vs_10k    67,823       85,244             17,421   25.69%   x 0.80
 btree::set::intersection_100_neg_vs_100_pos    17           27                     10   58.82%   x 0.63
 btree::set::intersection_100_neg_vs_10k_pos    17           32                     15   88.24%   x 0.53
 btree::set::intersection_100_pos_vs_100_neg    17           27                     10   58.82%   x 0.63
 btree::set::intersection_100_pos_vs_10k_neg    17           32                     15   88.24%   x 0.53
 btree::set::intersection_10k_neg_vs_100_pos    17           30                     13   76.47%   x 0.57
 btree::set::intersection_10k_neg_vs_10k_pos    18           33                     15   83.33%   x 0.55
 btree::set::intersection_10k_pos_vs_100_neg    17           31                     14   82.35%   x 0.55
 btree::set::intersection_10k_pos_vs_10k_neg    18           33                     15   83.33%   x 0.55
 btree::set::intersection_random_100_vs_100     635          728                    93   14.65%   x 0.87
 btree::set::intersection_random_100_vs_10k     2,129        2,567                 438   20.57%   x 0.83
 btree::set::intersection_random_10k_vs_100     2,121        2,706                 585   27.58%   x 0.78
 btree::set::intersection_staggered_100_vs_100  617          790                   173   28.04%   x 0.78
 btree::set::intersection_staggered_100_vs_10k  2,031        2,502                 471   23.19%   x 0.81
 btree::set::intersection_staggered_10k_vs_10k  60,891       75,550             14,659   24.07%   x 0.81
 btree::set::is_subset_100_vs_10k               1,185        1,445                 260   21.94%   x 0.82
 btree::set::is_subset_10k_vs_10k               59,529       52,343             -7,186  -12.07%   x 1.14

And it has been a struggle to get gdb introspection to work, one that apparently makes no impression on CI.