Improve onEach templates.

Add new functions to reference API.
Add tests for onEach
  #KT-8220

libraries/stdlib/test/collections/SequenceTest.kt

@@ -87,6 +87,22 @@ public class SequenceTest {
         assertEquals(listOf("f", "ba"), indexed)
     }
 
+    @Test
+    fun onEach() {
+        var count = 0
+        val data = sequenceOf("foo", "bar")
+        val newData = data.onEach { count += it.length }
+        assertFalse(data === newData)
+        assertEquals(0, count, "onEach should be executed lazily")
+
+        data.forEach {  }
+        assertEquals(0, count, "onEach should be executed only when resulting sequence is iterated")
+
+        val sum = newData.sumBy { it.length }
+        assertEquals(sum, count)
+    }
+
+
     @Test fun filterAndTakeWhileExtractTheElementsWithinRange() {
         assertEquals(listOf(144, 233, 377, 610, 987), fibonacci().filter { it > 100 }.takeWhile { it < 1000 }.toList())
     }