Have buffers store marks ranges.

This is very WIP.  The intent is for these to be used for e.g.
cursors, screen positions, etc.  The intent is for the buffer
itself to keep them synced properly with the actual text on edits,
although this is not yet implemented.

sub_crates/backend/src/buffer.rs

@@ -5,6 +5,17 @@ use ropey::Rope;
 pub struct Buffer {
     pub is_dirty: bool, // Is this buffer currently out of sync with disk.
     pub text: Rope,     // The actual text content.
+
+    // Sets of marked ranges to be used for various purposes.
+    //
+    // Each individual mark consists of a head and a tail.  The ordering of the
+    // head and tail is unspecified: either the head or the tail may come first
+    // in the text.  Both head and tail are specified in absolute indices.
+    //
+    // Within sets, marks cannot overlap or abutt, and they must be ordered.
+    // Overlapping or abutting marks will be merged, and out-of-order marks
+    // will be removed.
+    pub mark_sets: Vec<Vec<(usize, usize)>>,
 }
 
 impl Buffer {
@@ -12,16 +23,43 @@ impl Buffer {
         Buffer {
             is_dirty: false,
             text: text,
+            mark_sets: Vec::new(),
         }
     }
 
-    pub fn insert(&mut self, text: &str, char_idx: usize) {
+    // Replaces the given range of chars with the given text.
-        self.text.insert(char_idx, text);
+    pub fn edit(&mut self, char_idx_range: (usize, usize), text: &str) {
         self.is_dirty = true;
+
+        // Get the range, properly ordered.
+        let (start, end) = if char_idx_range.0 < char_idx_range.1 {
+            (char_idx_range.0, char_idx_range.1)
+        } else {
+            (char_idx_range.1, char_idx_range.0)
+        };
+
+        // Do removal if needed.
+        if start != end {
+            self.text.remove(start..end);
+        }
+
+        // Do insertion if needed.
+        if !text.is_empty() {
+            self.text.insert(start, text);
+        }
     }
 
-    pub fn remove(&mut self, char_start: usize, char_end: usize) {
+    /// Creates a new empty mark set, and returns the set index.
-        self.text.remove(char_start..char_end);
+    pub fn add_mark_set(&mut self) -> usize {
-        self.is_dirty = true;
+        self.mark_sets.push(vec![(0, 0)]);
+        return self.mark_sets.len() - 1;
+    }
+
+    pub fn insert_new_mark(&mut self, _set_idx: usize, _mark: (usize, usize)) {
+        todo!()
+    }
+
+    pub fn clear_mark_set(&mut self, set_idx: usize) {
+        self.mark_sets[set_idx].clear();
     }
 }