i_tree allowed out-of-bounds access through safe public node accessors

Affected versions of `i_tree` exposed safe public `Tree::node` and `Tree::mut_node` methods in the public `tree` module. These methods accepted an arbitrary `u32` index and passed it directly to `Vec::get_unchecked` / `get_unchecked_mut` on the internal node buffer, without validating that the index was in bounds.

Because these methods were safe and public, a caller could pass an out-of-bounds index without writing any `unsafe` code, producing an out-of-bounds shared or mutable reference and triggering undefined behavior.

Starting with `0.10.0` the crate was restructured and these accessors are no longer reachable from outside the crate.