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).

Author: cammeresi

library/alloc/src/collections/btree/node.rs

@@ -224,7 +224,10 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::Leaf> {
     }
 
     fn from_new_leaf<A: Allocator + Clone>(leaf: Box<LeafNode<K, V>, A>) -> Self {
-        NodeRef { height: 0, node: NonNull::from(Box::leak(leaf)), _marker: PhantomData }
+        let (leaf, _alloc) = Box::into_raw_with_allocator(leaf);
+        // SAFETY: the node was just allocated.
+        let node = unsafe { NonNull::new_unchecked(leaf) };
+        NodeRef { height: 0, node, _marker: PhantomData }
     }
 }
 
@@ -242,7 +245,10 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::Internal> {
         height: usize,
     ) -> Self {
         debug_assert!(height > 0);
-        let node = NonNull::from(Box::leak(internal)).cast();
+        let (internal, _alloc) = Box::into_raw_with_allocator(internal);
+        // SAFETY: the node was just allocated.
+        let internal = unsafe { NonNull::new_unchecked(internal) };
+        let node = internal.cast();
         let mut this = NodeRef { height, node, _marker: PhantomData };
         this.borrow_mut().correct_all_childrens_parent_links();
         this