Start work on exposing APIs for node descendant counts and indices

lib/binding_rust/lib.rs

@@ -1121,6 +1121,12 @@ impl<'tree> Node<'tree> {
         Self::new(unsafe { ffi::ts_node_prev_named_sibling(self.0) })
     }
 
+    /// Get the node's number of descendants, including one for the node itself.
+    #[doc(alias = "ts_node_descendant_count")]
+    pub fn descendant_count(&self) -> usize {
+        unsafe { ffi::ts_node_descendant_count(self.0) as usize }
+    }
+
     /// Get the smallest node within this node that spans the given range.
     #[doc(alias = "ts_node_descendant_for_byte_range")]
     pub fn descendant_for_byte_range(&self, start: usize, end: usize) -> Option<Self> {
@@ -1287,6 +1293,16 @@ impl<'a> TreeCursor<'a> {
         return unsafe { ffi::ts_tree_cursor_goto_next_sibling(&mut self.0) };
     }
 
+    /// Move the cursor to the node that is the nth descendant of
+    /// the original node that the cursor was constructed with, where
+    /// zero represents the original node itself.
+    #[doc(alias = "ts_tree_cursor_goto_descendant")]
+    pub fn goto_descendant(&mut self, descendant_index: usize) {
+        return unsafe {
+            ffi::ts_tree_cursor_goto_descendant(&mut self.0, descendant_index as u32)
+        };
+    }
+
     /// Move this cursor to the first child of its current node that extends beyond
     /// the given byte offset.
     ///