stdlib: add flatten()

libraries/stdlib/test/collections/IterableTests.kt

@@ -164,6 +164,11 @@ abstract class IterableTests<T : Iterable<String>>(val data: T, val empty: T) {
         assertEquals(listOf(3, 3), lengths)
     }
 
+    Test
+    fun flatten() {
+        assertEquals(listOf(0, 1, 2, 3, 0, 1, 2, 3), data.map { 0..it.length() }.flatten())
+    }
+
     Test
     fun mapIndexed() {
         val shortened = data.mapIndexed {(index, value) -> value.substring(0..index) }