sum_tree: Store context on cursor (#34904)

This gets rid of the need to pass context to all cursor functions. In
practice context is always immutable when interacting with cursors.

A nicety of this is in the follow-up PR we will be able to implement
Iterator for all Cursors/filter cursors (hell, we may be able to get rid
of filter cursor altogether, as it is just a custom `filter` impl on
iterator trait).
Release Notes:

- N/A

crates/editor/src/git/blame.rs

@@ -296,7 +296,7 @@ impl GitBlame {
             let row = info
                 .buffer_row
                 .filter(|_| info.buffer_id == Some(buffer_id))?;
-            cursor.seek_forward(&row, Bias::Right, &());
+            cursor.seek_forward(&row, Bias::Right);
             cursor.item()?.blame.clone()
         })
     }
@@ -389,7 +389,7 @@ impl GitBlame {
                 }
             }
 
-            new_entries.append(cursor.slice(&edit.old.start, Bias::Right, &()), &());
+            new_entries.append(cursor.slice(&edit.old.start, Bias::Right), &());
 
             if edit.new.start > new_entries.summary().rows {
                 new_entries.push(
@@ -401,7 +401,7 @@ impl GitBlame {
                 );
             }
 
-            cursor.seek(&edit.old.end, Bias::Right, &());
+            cursor.seek(&edit.old.end, Bias::Right);
             if !edit.new.is_empty() {
                 new_entries.push(
                     GitBlameEntry {
@@ -412,7 +412,7 @@ impl GitBlame {
                 );
             }
 
-            let old_end = cursor.end(&());
+            let old_end = cursor.end();
             if row_edits
                 .peek()
                 .map_or(true, |next_edit| next_edit.old.start >= old_end)
@@ -421,18 +421,18 @@ impl GitBlame {
                     if old_end > edit.old.end {
                         new_entries.push(
                             GitBlameEntry {
-                                rows: cursor.end(&()) - edit.old.end,
+                                rows: cursor.end() - edit.old.end,
                                 blame: entry.blame.clone(),
                             },
                             &(),
                         );
                     }
 
-                    cursor.next(&());
+                    cursor.next();
                 }
             }
         }
-        new_entries.append(cursor.suffix(&()), &());
+        new_entries.append(cursor.suffix(), &());
         drop(cursor);
 
         self.buffer_snapshot = new_snapshot;