Rollup merge of #146859 - cammeresi:btree-alloc-20250920, r=joboet

BTreeMap: Don't leak allocators when initializing nodes

Memory was allocated via `Box::leak` and thence intended to be tracked and deallocated manually, but the allocator was also leaked, not tracked, and never dropped.  Now it is dropped immediately.

According to my reading of the `Allocator` trait, if a copy of the allocator remains live, then its allocations must remain live.  Since the B-tree has a copy of the allocator that will only be dropped after the nodes, it's safe to not store the allocator in each node (which would be a much more intrusive change).

Fixes: rust-lang/rust#106203

Author: matthiaskrgr