toString() method for data classes and fake implementation for hashCode/equals

compiler/testData/codegen/dataClasses/tostring/mixedParams.kt

@@ -0,0 +1,7 @@
+data class A(var x: Int, y: Int, val z: Int?)
+
+fun box(): String {
+    val a = A(1, 2, null)
+    if("$a" != "A{x=1, z=null}") return "$a"
+    return "OK"
+}