directly drop the subtree instead of traversing every element when BTreemap dropping

[ywxt]

When both types of BTreeMap keys and values don't have destructors, currently it converts the tree to an iterator to traverse while dropping. However it's unnecessary. We instead can deallocate nodes of the tree directly.

Results on aarch64:
btree::map

benchmark	baseline ns/iter	new ns/iter	change
clone_fat_val_100	10849.14	7433.38	-31.5%
clone_fat_val_100_and_clear	11021.05	7606.91	-31.0%
clone_fat_val_100_and_drain_all	57652.20	55026.07	-4.6%
clone_fat_val_100_and_drain_half	27258.25	23993.80	-12.0%
clone_fat_val_100_and_into_iter	15149.81	12289.52	-18.9%
clone_fat_val_100_and_pop_all	56717.62	54045.12	-4.7%
clone_fat_val_100_and_remove_all	64333.00	61907.57	-3.8%
clone_fat_val_100_and_remove_half	28440.43	25290.79	-11.1%
clone_slim_100	1978.26	687.38	-65.3%
clone_slim_100_and_clear	1908.35	687.58	-64.0%
clone_slim_100_and_drain_all	3983.43	4244.24	+6.5%
clone_slim_100_and_drain_half	4241.43	3551.33	-16.3%
clone_slim_100_and_into_iter	1805.39	1753.37	-2.9%
clone_slim_100_and_pop_all	3243.54	3247.17	+0.1%
clone_slim_100_and_remove_all	5203.97	5176.58	-0.5%
clone_slim_100_and_remove_half	2399.04	1818.85	-24.2%
clone_slim_10k	205051.50	74942.50	-63.5%
clone_slim_10k_and_clear	197575.50	74994.75	-62.0%
clone_slim_10k_and_drain_all	425268.00	424679.50	-0.1%
clone_slim_10k_and_drain_half	408679.00	358468.67	-12.3%
clone_slim_10k_and_into_iter	184573.50	180805.00	-2.0%
clone_slim_10k_and_pop_all	369476.00	370334.50	+0.2%
clone_slim_10k_and_remove_all	582219.44	581719.69	-0.1%
clone_slim_10k_and_remove_half	424686.50	373048.00	-12.2%
from_iter_rand_100	1832.99	1086.70	-40.7%
from_iter_rand_10_000	304447.00	231011.33	-24.1%
from_iter_seq_100	2045.17	750.12	-63.3%
from_iter_seq_10_000	211268.33	80514.29	-61.9%

btree::set

benchmark	baseline ns/iter	new ns/iter	change
clone_100	1913.17	605.95	-68.3%
clone_100_and_clear	1813.35	608.09	-66.5%
clone_100_and_drain_all	3960.52	3965.28	+0.1%
clone_100_and_drain_half	4045.63	3408.90	-15.7%
clone_100_and_into_iter	1676.32	1600.11	-4.5%
clone_100_and_pop_all	2582.26	2562.19	-0.8%
clone_100_and_remove_all	4574.45	4520.84	-1.2%
clone_100_and_remove_half	2107.02	1533.22	-27.2%
clone_10k	192719.50	65933.38	-65.8%
clone_10k_and_clear	184669.60	66101.87	-64.2%
clone_10k_and_drain_all	418832.00	417920.00	-0.2%
clone_10k_and_drain_half	400446.00	348149.50	-13.1%
clone_10k_and_into_iter	170359.60	163481.80	-4.0%
clone_10k_and_pop_all	306021.67	307441.67	+0.5%
clone_10k_and_remove_all	510750.88	511582.56	+0.2%
clone_10k_and_remove_half	385672.00	337072.33	-12.6%

r? @Amanieu