=OrderedSet.filter Attempt to optimize filter impl

Benchmarks/Benchmarks/OrderedSetBenchmarks.swift

@@ -269,6 +269,25 @@ extension Benchmark {
       }
     }
 
+    for percent in [0, 25, 50, 75, 100] {
+      self.add(
+        title: "OrderedSet<Int> filter keeping \(percent)%",
+        input: [Int].self
+      ) { input in
+        let set = OrderedSet(input)
+        return { timer in
+          var r: OrderedSet<Int>!
+          timer.measure {
+            r = set.filter { $0 % 100 < percent }
+          }
+          let div = input.count / 100
+          let rem = input.count % 100
+          precondition(r.count == percent * div + Swift.min(rem, percent))
+          blackHole(r)
+        }
+      }
+    }
+
     let overlaps: [(String, (Int) -> Int)] = [
       ("0%",   { c in c }),
       ("25%",  { c in 3 * c / 4 }),

Sources/OrderedCollections/OrderedSet/OrderedSet.swift

@@ -508,13 +508,15 @@ extension OrderedSet {
   /// - Complexity: O(`count`)
   @inlinable
   public func filter(
-          _ isIncluded: (Element) throws -> Bool
+    _ isIncluded: (Element) throws -> Bool
   ) rethrows -> Self {
-    var result: OrderedSet = Self(minimumCapacity: _minimumCapacity)
-    for element in self where try isIncluded(element) {
-      result._appendNew(element)
+    try _UnsafeBitset.withTemporaryBitset(capacity: self.count) { bitset in
+      for i in _elements.indices where try isIncluded(_elements[i]) {
+        bitset.insert(i)
+      }
+      let result = self._extractSubset(using: bitset)
+      result._checkInvariants()
+      return result
     }
-    result._checkInvariants()
-    return result
   }
 }