Start on a new FragmentList

Here I'm exploring a new approach to the project-wide diagnostics view that can exactly mirror the contents of cargo check. The `FragmentList` composes an arbitrary list of fragments from other buffers and presents them as if they were a single buffer.

crates/text/src/text.rs

@@ -49,7 +49,7 @@ pub struct Buffer {
     subscriptions: Vec<Weak<Mutex<Vec<Patch<usize>>>>>,
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct Snapshot {
     visible_text: Rope,
     deleted_text: Rope,