feat(tree): enhance tree structure with new features and optimizations

- Added support for `maybe_uninit_slice` feature in `lib.rs`
- Introduced `Borrow` and `Ordering` imports in `tree.rs`
- Refactored marker structs and renamed `KV` to `Value`, `Immutable` to `Immut`
- Updated `LeafNode` to use `Option<MaybeUninit<V>>` for value storage
- Simplified `NodeRef` type parameters and added `Sync` and `Send` implementations
- Enhanced `Handle` with `new_value` method for value handling
- Added methods for parent link correction and child link management
- Implemented `search` module for tree traversal and key lookup
- Introduced `entry` module for managing vacant and occupied entries
- Added `DormantMutRef` utility for safe mutable reference reborrowing

src/lib.rs

@@ -1,7 +1,7 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(
     feature = "nightly",
-    feature(strict_provenance_atomic_ptr, box_vec_non_null)
+    feature(strict_provenance_atomic_ptr, box_vec_non_null, maybe_uninit_slice)
 )]
 #![cfg_attr(feature = "transposed-option", feature(try_trait_v2))]