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

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

@@ -0,0 +1,11 @@
+data class A(var string: String)
+
+fun box(): String {
+    val a = A("Fail")
+    if(a.toString() != "A{string=Fail}") return "fail"
+
+    a.string = "OK"
+    if("$a" != "A{string=OK}") return a.toString()
+
+    return "OK"
+}