Introduce new overloads of flatMap and flatMapTo

- Sequence<T>.flatMap((T) -> Iterable<R>)
- Iterable<T>.flatMap((T) -> Sequence<R>)
- Array<T>.flatMap((T) -> Sequence<R>)
- Map.flatMap((Entry) -> Sequence<R>)

KT-34506

libraries/stdlib/test/collections/ArraysTest.kt

@@ -12,6 +12,7 @@ import test.assertTypeEquals
 import test.collections.behaviors.*
 import test.comparisons.STRING_CASE_INSENSITIVE_ORDER
 import test.text.isAsciiLetter
+import kotlin.math.exp
 import kotlin.test.*
 import kotlin.random.Random
 
@@ -1756,6 +1757,15 @@ class ArraysTest {
         assertEquals(listOf(2), arrayOf("a", null, "test").mapIndexedNotNull { index, it -> it?.run { if (index != 0) length / index else null  } })
     }
 
+    @Test fun flatMap() {
+        val data = arrayOf(arrayOf(1, 2, 3), arrayOf(4, 5, 6))
+        val result1 = data.flatMap { it.asList() }
+        val result2 = data.flatMap { it.asSequence() }
+        val expected = (data[0] + data[1]).toList()
+        assertEquals(expected, result1)
+        assertEquals(expected, result2)
+    }
+
     @Test fun flattenArray() {
         val arr1: Array<Array<Int>> = arrayOf(arrayOf(1, 2, 3), arrayOf(4, 5, 6))
         val arr2: Array<out Array<Int>> = arr1