Easy way to respect BTreeMap's minimum node length

[ssomers]

Resolves #74834 the easy way.

Benchmarks (which are all biased/realistic, inserting keys in ascending order) say:

benchcmp r0 r3 --threshold 10
 name                                           r0 ns/iter  r3 ns/iter  diff ns/iter   diff %  speedup
 btree::map::clone_fat_100_and_drain_all        156,137     178,180           22,043   14.12%   x 0.88
 btree::map::clone_fat_100_and_drain_half       121,160     142,412           21,252   17.54%   x 0.85
 btree::map::clone_fat_100_and_pop_all          176,110     196,380           20,270   11.51%   x 0.90
 btree::map::clone_fat_100_and_remove_all       215,075     237,690           22,615   10.51%   x 0.90
 btree::map::clone_fat_100_and_remove_half      142,377     164,035           21,658   15.21%   x 0.87
 btree::map::clone_fat_val_100_and_drain_half   60,372      70,638            10,266   17.00%   x 0.85
 btree::map::clone_fat_val_100_and_remove_half  64,124      75,858            11,734   18.30%   x 0.85
 btree::map::clone_slim_100_and_clear           2,183       2,838                655   30.00%   x 0.77
 btree::map::clone_slim_100_and_drain_all       3,652       4,350                698   19.11%   x 0.84
 btree::map::clone_slim_100_and_drain_half      3,320       4,443              1,123   33.83%   x 0.75
 btree::map::clone_slim_100_and_into_iter       2,154       2,836                682   31.66%   x 0.76
 btree::map::clone_slim_100_and_pop_all         3,372       4,051                679   20.14%   x 0.83
 btree::map::clone_slim_100_and_remove_all      5,111       5,820                709   13.87%   x 0.88
 btree::map::clone_slim_100_and_remove_half     3,259       4,198                939   28.81%   x 0.78
 btree::map::clone_slim_10k                     244,160     276,820           32,660   13.38%   x 0.88
 btree::map::clone_slim_10k_and_clear           244,032     277,426           33,394   13.68%   x 0.88
 btree::map::clone_slim_10k_and_drain_all       384,035     425,290           41,255   10.74%   x 0.90
 btree::map::clone_slim_10k_and_drain_half      383,575     439,660           56,085   14.62%   x 0.87
 btree::map::clone_slim_10k_and_into_iter       241,980     274,260           32,280   13.34%   x 0.88
 btree::map::iter_0                             1,733       1,487               -246  -14.20%   x 1.17
 btree::map::iter_100                           2,714       3,707                993   36.59%   x 0.73
 btree::map::iter_10k                           3,728       4,194                466   12.50%   x 0.89
 btree::map::iter_1m                            5,570       6,581              1,011   18.15%   x 0.85
 btree::map::range_unbounded_unbounded          28,426      36,604             8,178   28.77%   x 0.78
 btree::map::range_unbounded_vs_iter            28,808      37,503             8,695   30.18%   x 0.77
 btree::set::clone_100_and_drain_half           2,935       3,458                523   17.82%   x 0.85
 btree::set::clone_100_and_remove_half          2,869       3,252                383   13.35%   x 0.88
 btree::set::clone_10k                          205,255     232,898           27,643   13.47%   x 0.88
 btree::set::clone_10k_and_clear                204,650     231,412           26,762   13.08%   x 0.88
 btree::set::clone_10k_and_drain_all            319,003     354,305           35,302   11.07%   x 0.90
 btree::set::clone_10k_and_drain_half           332,200     374,400           42,200   12.70%   x 0.89
 btree::set::clone_10k_and_into_iter            204,652     229,792           25,140   12.28%   x 0.89
 btree::set::clone_10k_and_pop_all              318,360     351,400           33,040   10.38%   x 0.91

r? @Mark-Simulacrum

[ssomers]

Not a work in progress, but I don't think the gloomy performance prediction makes it a genuine alternative to #75105.