Make DurationUnit not a typealias of TimeUnit on JVM

Provide conversion functions between DurationUnit and TimeUnit

libraries/stdlib/jvm/test/time/DurationUnitJVMTest.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package test.time
+
+import java.util.concurrent.TimeUnit
+import kotlin.test.*
+import kotlin.time.*
+
+@OptIn(ExperimentalTime::class)
+class DurationUnitJVMTest {
+    @Test
+    fun conversionFromTimeUnit() {
+        for (unit in DurationUnit.values()) {
+            val timeUnit = unit.toTimeUnit()
+            assertEquals(unit.name, timeUnit.name)
+            assertEquals(unit, timeUnit.toDurationUnit())
+        }
+
+        for (timeUnit in TimeUnit.values()) {
+            val unit = timeUnit.toDurationUnit()
+            assertEquals(timeUnit.name, unit.name)
+            assertEquals(timeUnit, unit.toTimeUnit())
+        }
+    }
+}