BTreeMap::entry: Avoid allocating if no insertion This PR allows the `VacantEntry` to borrow from an empty tree with no root, and to lazily allocate a new root node when the user calls `.insert(value)`.
BTreeMap::entry: Avoid allocating if no insertion This PR allows the `VacantEntry` to borrow from an empty tree with no root, and to lazily allocate a new root node when the user calls `.insert(value)`.