Changing API name; Adding unit test and Rust bindings

cli/src/tests/node_test.rs

@@ -248,6 +248,25 @@ fn test_node_parent_of_child_by_field_name() {
     );
 }
 
+#[test]
+fn test_node_field_name_for_child() {
+    let mut parser = Parser::new();
+    parser.set_language(get_language("c")).unwrap();
+    let tree = parser.parse("x + y;", None).unwrap();
+    let translation_unit_node = tree.root_node();
+    let binary_expression_node = translation_unit_node
+                                 .named_child(0)
+                                 .unwrap()
+                                 .named_child(0)
+                                 .unwrap();
+
+    assert_eq!(binary_expression_node.field_name_for_child(0), Some("left"));
+    assert_eq!(binary_expression_node.field_name_for_child(1), Some("operator"));
+    assert_eq!(binary_expression_node.field_name_for_child(2), Some("right"));
+		// Negative test - Not a valid child index
+    assert_eq!(binary_expression_node.field_name_for_child(3), None);
+}
+
 #[test]
 fn test_node_child_by_field_name_with_extra_hidden_children() {
     let mut parser = Parser::new();