Provide distinctBy(keySelector) method for collections and sequences.

Implement distinct() and toMutableSet() for sequences.
Breaking change: distinct() now returns List instead of Set.

#KT-5834 Fixed
#KT-6063 Fixed

libraries/stdlib/test/collections/SetOperationsTest.kt

@@ -5,8 +5,13 @@ import org.junit.Test as test
 
 class SetOperationsTest {
     test fun distinct() {
-        assertEquals(listOf(1, 3, 5), listOf(1, 3, 3, 1, 5, 1, 3).distinct().toList())
-        assertTrue(listOf<Int>().distinct().none())
+        assertEquals(listOf(1, 3, 5), listOf(1, 3, 3, 1, 5, 1, 3).distinct())
+        assertTrue(listOf<Int>().distinct().isEmpty())
+    }
+
+    test fun distinctBy() {
+        assertEquals(listOf("some", "cat", "do"), arrayOf("some", "case", "cat", "do", "dog", "it").distinctBy { it.length() })
+        assertTrue(charArrayOf().distinctBy { it }.isEmpty())
     }
 
     test fun union() {