Rename Clock to TimeSource, ClockMark to TimeMark

Step 2: rename classes and interfaces.
Provide deprecated typealiases to smooth migration.

libraries/stdlib/test/time/TimeMarkTest.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2010-2020 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.
+ */
+@file:UseExperimental(ExperimentalTime::class)
+package test.time
+
+import kotlin.test.*
+import kotlin.time.*
+
+class TimeMarkTest {
+
+    @Test
+    fun adjustment() {
+        val timeSource = TestTimeSource()
+
+        fun TimeMark.assertHasPassed(hasPassed: Boolean) {
+            assertEquals(!hasPassed, this.hasNotPassedNow(), "Expected mark in the future")
+            assertEquals(hasPassed, this.hasPassedNow(), "Expected mark in the past")
+
+            assertEquals(!hasPassed, this.elapsedNow() < Duration.ZERO, "Mark elapsed: ${this.elapsedNow()}, expected hasPassed: $hasPassed")
+        }
+
+        val mark = timeSource.markNow()
+        val markFuture1 = (mark + 1.milliseconds).apply { assertHasPassed(false) }
+        val markFuture2 = (mark - (-1).milliseconds).apply { assertHasPassed(false) }
+
+        val markPast1 = (mark - 1.milliseconds).apply { assertHasPassed(true) }
+        val markPast2 = (markFuture1 + (-2).milliseconds).apply { assertHasPassed(true) }
+
+        timeSource += 500_000.nanoseconds
+
+        val elapsed = mark.elapsedNow()
+        val elapsedFromFuture = elapsed - 1.milliseconds
+        val elapsedFromPast = elapsed + 1.milliseconds
+
+        assertEquals(0.5.milliseconds, elapsed)
+        assertEquals(elapsedFromFuture, markFuture1.elapsedNow())
+        assertEquals(elapsedFromFuture, markFuture2.elapsedNow())
+
+        assertEquals(elapsedFromPast, markPast1.elapsedNow())
+        assertEquals(elapsedFromPast, markPast2.elapsedNow())
+
+        markFuture1.assertHasPassed(false)
+        markPast1.assertHasPassed(true)
+
+        timeSource += 1.milliseconds
+
+        markFuture1.assertHasPassed(true)
+        markPast1.assertHasPassed(true)
+
+    }
+}