Implement FoldMap's folds using a SumTree

This required passing a `Context` object to `Summary` and introducing a
new `SeekDimension` trait that allows comparing two dimensions and pass
an additional context object.

zed/src/editor/buffer/mod.rs

@@ -2103,6 +2103,8 @@ impl sum_tree::Item for Fragment {
 }
 
 impl sum_tree::Summary for FragmentSummary {
+    type Context = ();
+
     fn add_summary(&mut self, other: &Self) {
         self.text_summary += &other.text_summary;
         debug_assert!(self.max_fragment_id <= other.max_fragment_id);
@@ -2167,6 +2169,8 @@ impl sum_tree::Item for InsertionSplit {
 }
 
 impl sum_tree::Summary for InsertionSplitSummary {
+    type Context = ();
+
     fn add_summary(&mut self, other: &Self) {
         self.extent += other.extent;
     }

zed/src/editor/buffer/text.rs

@@ -59,6 +59,8 @@ pub struct TextSummary {
 }
 
 impl sum_tree::Summary for TextSummary {
+    type Context = ();
+
     fn add_summary(&mut self, other: &Self) {
         *self += other;
     }