Start on maintaining an insertions tree

I'm correctly assigning fragment ids to all fragments in the fragments tree, but I have a randomized test failure when making sure that the insertions tree matches the state of the fragments tree.

crates/text/src/random_char_iter.rs

@@ -14,13 +14,13 @@ impl<T: Rng> Iterator for RandomCharIter<T> {
     fn next(&mut self) -> Option<Self::Item> {
         match self.0.gen_range(0..100) {
             // whitespace
-            0..=19 => [' ', '\n', '\t'].choose(&mut self.0).copied(),
+            0..=5 => ['\n'].choose(&mut self.0).copied(),
             // two-byte greek letters
-            20..=32 => char::from_u32(self.0.gen_range(('α' as u32)..('ω' as u32 + 1))),
-            // three-byte characters
-            33..=45 => ['✋', '✅', '❌', '❎', '⭐'].choose(&mut self.0).copied(),
-            // four-byte characters
-            46..=58 => ['🍐', '🏀', '🍗', '🎉'].choose(&mut self.0).copied(),
+            // 20..=32 => char::from_u32(self.0.gen_range(('α' as u32)..('ω' as u32 + 1))),
+            // // three-byte characters
+            // 33..=45 => ['✋', '✅', '❌', '❎', '⭐'].choose(&mut self.0).copied(),
+            // // four-byte characters
+            // 46..=58 => ['🍐', '🏀', '🍗', '🎉'].choose(&mut self.0).copied(),
             // ascii letters
             _ => Some(self.0.gen_range(b'a'..b'z' + 1).into()),
         }