Use FunctionImplN instead of FunctionN in codegen

Superclass of closures should now be FunctionImplN instead of FunctionN. Since
these -Impl classes are needed only in JVM, the corresponding descriptors and
types are created in the back-end only.

compiler/testData/codegen/box/classes/kt1535.kt

@@ -1,9 +1,9 @@
-class Works() : jet.Function0<Object>() {
+class Works() : jet.Function0<Object> {
     public override fun invoke():Object {
       return "Works" as Object
     }
 }
-class Broken() : jet.Function0<String>() {
+class Broken() : jet.Function0<String> {
     public override fun invoke():String {
       return "Broken"
     }