fix: various tree.rs issues and unit tests

[distractedm1nd]

Set base as cleanup for now to make diff viewing easier.

tree.rs

Fixes:

• double_tree_size panics when called from insert_node
• generate_non_membership_proof returns a non-membership proof for members
• update_node allows for modification of all fields, instead of just the value
• missing API function to find node index by label without indirection

Unit Tests

• test_new_with_size
• test_membership_proofs
• test_find_node_index
• test_insert_node
• test_update_node
• test_find_leaf_by_label
• test_double_tree_size
• test_insert_node_doubles_tree_size
• new
• rehash_inner_nodes
• rebuild_tree_from_leaves
• calculate_root
• get_root
• get_commitment
• find_leaf_by_label (Although used in tests, no direct test aims at this method)
• generate_membership_proof (Though used in a test, it may lack a direct-focused test)
• generate_non_membership_proof (Though used, direct-focused tests might be beneficial)
• generate_proof (No corresponding method explicitly named, but related proof methods exist and have been partially tested)
• insert_node (Tested, but more in-depth cases could be beneficial, e.g., edge cases)
• update_node (Tested, but variations or edge cases might not be covered)
• set_left_sibling_status_for_nodes (Utility function used but not directly tested)
• resort_nodes_by_input_order

node.rs

Unit Tests

• test_leaf_node_creation
• test_inner_node_creation
• test_node_default
• test_node_is_active
• test_node_set_left_sibling
• test_leaf_node_activation
• test_update_next_pointer
• InnerNode::new - Implicitly tested through test_inner_node_creation, but no direct tests on certain edge cases (e.g., handling of extreme index values).
• LeafNode::new - Implicitly tested through test_leaf_node_creation, however direct tests for hash generation logic or edge cases are missing.
• Node::new_leaf - No direct unit test, but its outcome is tested indirectly within other tests.
• Node::new_inner - Similarly, no direct test exists, but its result is implicitly checked.
• Node::get_hash - Not directly tested, though its correctness is inferred through other tests.
• Node::is_left_sibling - While test_node_set_left_sibling verifies setting this attribute, direct testing of initial values or changes over operations is missing.
• Node::is_active - Covered by test_node_is_active, but scenarios of changing activity status aren't tested.
• Node::set_left_sibling_value - Tested by test_node_set_left_sibling.
• Node::set_node_active - Covered by test_leaf_node_activation.
• Node::add_left and Node::add_right - The functionalities of these methods haven't been directly tested. They play a crucial role in tree restructuring but lack individual verification.
• Node::update_next_pointer - Directly tested by test_update_next_pointer.
• Node::generate_hash - Absence of direct tests, particularly concerning the accurate composition and subsequent hash generation for both inner and leaf nodes.