Allow seeking SumTree cursor by multiple seek types per dimension

Also, remove the cursor's sum_dimension. Replace it with a
blanket implementation of Dimension for two-element tuples
of dimensions.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

zed/src/editor/buffer/operation_queue.rs

@@ -43,7 +43,7 @@ impl OperationQueue {
         clone
     }
 
-    pub fn cursor(&self) -> Cursor<Operation, (), ()> {
+    pub fn cursor(&self) -> Cursor<Operation, ()> {
         self.0.cursor()
     }
 }