[Rope] replaceSubrange: take a sequence, not a collection

Sources/RopeModule/Rope/Basics/Rope+Builder.swift

@@ -194,7 +194,7 @@ extension Rope {
         return
       }
       var item = item
-      if prefix.rebalance(nextNeighbor: &item) {
+      if (prefix.isUndersized || item.isUndersized), prefix.rebalance(nextNeighbor: &item) {
         self._prefix = prefix
         return
       }
@@ -221,7 +221,19 @@ extension Rope {
       guard rope._root != nil else { return }
       _insertBeforeTip(rope.root)
     }
-
+
+    @inlinable
+    public mutating func insertBeforeTip<S: Sequence<Element>>(_ items: __owned S) {
+      if S.self == Rope.self {
+        let items = _identityCast(items, to: Rope.self)
+        self.insertBeforeTip(items)
+      } else {
+        for item in items {
+          self.insertBeforeTip(item)
+        }
+      }
+    }
+
     @inlinable
     mutating func _insertBeforeTip(_ node: __owned Rope._Node) {
       defer { _invariantCheck() }

Sources/RopeModule/Rope/Operations/Rope+RemoveSubrange.swift

@@ -29,16 +29,14 @@ extension Rope {
   public mutating func replaceSubrange(
     _ bounds: Range<Int>,
     in metric: some RopeMetric<Element>,
-    with newElements: __owned some Collection<Element>
+    with newElements: __owned some Sequence<Element>
   ) {
     // FIXME: Implement insert(contentsOf:at:in:) and dispatch to it when bounds.isEmpty.
     // FIXME: Add fast path for replacing tiny ranges with tiny data.
     // FIXME: Add special cases if newElements is itself a _Rope etc.
     _invalidateIndices()
     var builder = builder(removing: bounds, in: metric)
-    for item in newElements {
-      builder.insertBeforeTip(item)
-    }
+    builder.insertBeforeTip(newElements)
     self = builder.finalize()
   }