Add channel benchmarks

benchmarks/src/jmh/kotlin/benchmarks/ChannelNanoBenchmarkConflated.kt

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+package benchmarks
+
+import kotlinx.coroutines.channels.*
+import org.openjdk.jmh.annotations.*
+import java.util.concurrent.*
+
+@Warmup(iterations = 30, time = 1)
+@Measurement(iterations = 30, time = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+@Fork(1)
+open class ChannelNanoBenchmarkConflated {
+    var channel: Channel<Int> = Channel(Channel.CONFLATED)
+
+    @Benchmark
+    fun trySend() {
+        channel.trySend(42)
+    }
+
+    @Benchmark
+    fun trySendTryReceive(): Int {
+        channel.trySend(42)
+        return channel.tryReceive().getOrThrow()
+    }
+}

benchmarks/src/jmh/kotlin/benchmarks/ChannelNanoBenchmarkUnlimited.kt

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+package benchmarks
+
+import kotlinx.coroutines.channels.*
+import org.openjdk.jmh.annotations.*
+import java.util.concurrent.*
+
+@Warmup(iterations = 30, time = 1)
+@Measurement(iterations = 30, time = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@Fork(1)
+open class ChannelNanoBenchmarkUnlimited {
+    @State(Scope.Benchmark)
+    open class PrefilledChannelState {
+        private val list = List(1_000_000) { it }
+
+        @Param(value = ["0", "10000", "100000", "1000000"])  // 0, 40, 400 KB, 4 MB
+        private var prefill = 0
+
+        lateinit var channel: Channel<Int>
+
+        @Setup(Level.Trial)
+        fun createPrefilledChannel() {
+            channel = Channel(Channel.UNLIMITED)
+            repeat(prefill) {
+                channel.trySend(list[it])
+            }
+        }
+    }
+
+    @Benchmark
+    fun trySendTryReceive(s: PrefilledChannelState): Int {
+        s.channel.trySend(42)
+        return s.channel.tryReceive().getOrThrow()
+    }
+}