Reverse iteration feature

[plar]

This PR introduces several enhancements and refactors to improve the tree traversal capabilities within the codebase, adding the ability to traverse in reverse order, refining API methods, and restructuring tests for better coverage and readability. Below are the main changes included in this PR:

Key Changes

Traversal Enhancements

• Added a new traversal option, tree.TraverseReverse, allowing iterations over nodes in reverse order.
• Modified the tree.ForEach and tree.ForEachPrefix methods to support optional traversal parameters.

Iterator Refactoring

• Introduced newTraverseFunc to replace traverseContext, which simplifies the traversal logic by using function types (traverseFunc) instead of struct-based contexts.
• Enhanced the iterator logic to support reverse traversals.

Test Suite Expansion

• Added tests for prefix-based traversal operations in both directions.
• Created new tests to verify the reverse traversal implementation for tree.ForEach, tree.ForEachPrefix, and tree.Iterator.

Overall Benefits

Performance

The introduction of reverse traversal provides a flexible and efficient means of traversing large datasets, reducing overhead in scenarios where reverse-order processing is needed.