integrate forward cursor into hashed post state cursors

crates/trie/trie/src/forward_cursor.rs

@@ -13,32 +13,38 @@ impl<'a, K, V> ForwardInMemoryCursor<'a, K, V> {
     pub const fn new(entries: &'a Vec<(K, V)>) -> Self {
         Self { entries, index: 0 }
     }
+
+    /// Returns `true` if the cursor is empty, regardless of its position.
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
 }
 
 impl<'a, K, V> ForwardInMemoryCursor<'a, K, V>
 where
-    K: PartialOrd,
+    K: PartialOrd + Copy,
+    V: Copy,
 {
     /// Advances the cursor forward while `comparator` returns `true` or until the collection is
     /// exhausted. Returns the first entry for which `comparator` returns `false` or `None`.
-    fn advance_while(&mut self, comparator: impl Fn(&K) -> bool) -> Option<&(K, V)> {
+    fn advance_while(&mut self, comparator: impl Fn(&K) -> bool) -> Option<(K, V)> {
         let mut entry = self.entries.get(self.index);
         while entry.map_or(false, |entry| comparator(&entry.0)) {
             self.index += 1;
             entry = self.entries.get(self.index);
         }
-        entry
+        entry.copied()
     }
 
     /// Returns the first entry from the current cursor position that's greater or equal to the
     /// provided key. This method advances the cursor forward.
-    pub fn seek(&mut self, key: &K) -> Option<&(K, V)> {
+    pub fn seek(&mut self, key: &K) -> Option<(K, V)> {
         self.advance_while(|k| k < key)
     }
 
     /// Returns the first entry from the current cursor position that's greater than the provided
     /// key. This method advances the cursor forward.
-    pub fn first_after(&mut self, key: &K) -> Option<&(K, V)> {
+    pub fn first_after(&mut self, key: &K) -> Option<(K, V)> {
         self.advance_while(|k| k <= key)
     }
 }