Introduce dedicated ZipWithIndexGatherer (#33)

src/main/java/com/pivovarit/gatherers/MoreGatherers.java

@@ -136,12 +136,6 @@ class State {
     }
 
     public static <T> Gatherer<T, ?, Map.Entry<Long, T>> zipWithIndex() {
-        return ofSequential(
-          AtomicLong::new,
-          ofGreedy((state, element, downstream) -> {
-              downstream.push(Map.entry(state.getAndIncrement(), element));
-              return true;
-          })
-        );
+        return new ZipWithIndexGatherer<>();
     }
 }

src/main/java/com/pivovarit/gatherers/ZipWithIndexGatherer.java

@@ -0,0 +1,22 @@
+package com.pivovarit.gatherers;
+
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
+import java.util.stream.Gatherer;
+
+record ZipWithIndexGatherer<T>() implements Gatherer<T, AtomicLong, Map.Entry<Long, T>> {
+
+    @Override
+    public Supplier<AtomicLong> initializer() {
+        return AtomicLong::new;
+    }
+
+    @Override
+    public Integrator<AtomicLong, T, Map.Entry<Long, T>> integrator() {
+        return Integrator.ofGreedy((state, element, downstream) -> {
+            downstream.push(Map.entry(state.getAndIncrement(), element));
+            return true;
+        });
+    }
+}