Add benchmarks for ByteIterator

Motivation:
Capture the performance characteristics of this primitive for various buffer implementations.

Modification:
Add a benchmark that iterate 4KiB buffers forwards, and backwards, on various buffer implementations.

Result:
Another aspect of the implementation covered by benchmarks.
Turns out the composite iterators a somewhat slow.

src/test/java/io/netty/buffer/api/benchmarks/ByteIterationBenchmark.java

@@ -0,0 +1,118 @@
+/*
+ * Copyright 2019 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.buffer.api.benchmarks;
+
+import io.netty.buffer.api.Allocator;
+import io.netty.buffer.api.Buf;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.TearDown;
+import org.openjdk.jmh.annotations.Warmup;
+
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.concurrent.TimeUnit;
+
+@Warmup(iterations = 10, time = 1)
+@Measurement(iterations = 10, time = 1)
+@Fork(value = 5, jvmArgsAppend = { "-XX:+UnlockDiagnosticVMOptions", "-XX:+DebugNonSafepoints" })
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+public class ByteIterationBenchmark {
+    private static final int SIZE = 4096;
+    @Param({"heap", "direct", "composite-heap", "composite-direct"})
+    public String type;
+
+    Allocator allocator;
+    private Buf buf;
+
+    @Setup
+    public void setUp() {
+        switch (type) {
+        case "heap":
+            allocator = Allocator.heap();
+            buf = allocator.allocate(SIZE);
+            break;
+        case "direct":
+            allocator = Allocator.direct();
+            buf = allocator.allocate(SIZE);
+            break;
+        case "composite-heap":
+            allocator = Allocator.heap();
+            try (var a = allocator.allocate(SIZE / 2);
+                 var b = allocator.allocate(SIZE / 2)) {
+                buf = allocator.compose(a, b);
+            }
+            break;
+        case "composite-direct":
+            allocator = Allocator.direct();
+            try (var a = allocator.allocate(SIZE / 2);
+                 var b = allocator.allocate(SIZE / 2)) {
+                buf = allocator.compose(a, b);
+            }
+            break;
+        default:
+            throw new IllegalArgumentException("Unknown buffer type: " + type + '.');
+        }
+        ThreadLocalRandom tlr = ThreadLocalRandom.current();
+        while (buf.writableBytes() > 7) {
+            buf.writeLong(tlr.nextLong());
+        }
+        while (buf.writableBytes() > 0) {
+            buf.writeByte((byte) tlr.nextInt());
+        }
+    }
+
+    @TearDown
+    public void tearDown() {
+        buf.close();
+        allocator.close();
+    }
+
+    @Benchmark
+    public long sum() {
+        var itr = buf.iterate();
+        long sum = 0;
+        while (itr.hasNextLong()) {
+            sum += itr.nextLong();
+        }
+        while (itr.hasNextByte()) {
+            sum += itr.nextByte();
+        }
+        return sum;
+    }
+
+    @Benchmark
+    public long sumReverse() {
+        var itr = buf.iterateReverse();
+        long sum = 0;
+        while (itr.hasNextLong()) {
+            sum += itr.nextLong();
+        }
+        while (itr.hasNextByte()) {
+            sum += itr.nextByte();
+        }
+        return sum;
+    }
+}