Suggest duration static factories instead of number extension properties

Introduce Duration companion functions to convert numbers to Duration.
Deprecate number extension properties and propose to use these
new functions instead.

libraries/stdlib/test/time/MeasureTimeTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.
  */
 
@@ -52,20 +52,20 @@ class MeasureTimeTest {
         val timeSource = TestTimeSource()
         val expectedNs = Random.nextLong(1_000_000_000L)
         val elapsed = timeSource.measureTime {
-            timeSource += expectedNs.nanoseconds
+            timeSource += Duration.nanoseconds(expectedNs)
         }
 
-        assertEquals(expectedNs.nanoseconds, elapsed)
+        assertEquals(Duration.nanoseconds(expectedNs), elapsed)
 
         val expectedResult: Long
 
         val (result, elapsed2) = timeSource.measureTimedValue {
-            timeSource += expectedNs.nanoseconds
+            timeSource += Duration.nanoseconds(expectedNs)
             expectedResult = expectedNs
             expectedNs
         }
 
         assertEquals(expectedResult, result)
-        assertEquals(result.nanoseconds, elapsed2)
+        assertEquals(Duration.nanoseconds(result), elapsed2)
     }
 }