Add an optional parameter to joinToString() and joinTo() functions: lambda function which maps the element to string.

#KT-5468 Fixed

libraries/stdlib/test/text/StringJVMTest.kt

@@ -240,6 +240,10 @@ class StringJVMTest {
         val data2 = "verylongstring".toList()
         val result2 = data2.joinToString("-", "[", "]", 11, "oops")
         assertEquals("[v-e-r-y-l-o-n-g-s-t-r-oops]", result2)
+
+        val data3 = "a1/b".toList()
+        val result3 = data3.joinToString() { it.toUpperCase().toString() }
+        assertEquals("A, 1, /, B", result3)
     }
 
     test fun join() {