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"
     }

compiler/testData/codegen/box/closures/closureOnTopLevel1.kt

@@ -7,7 +7,7 @@ val getter: String
 
 fun f() = { "OK" }()
 
-val obj = object : Function0<String>() {
+val obj = object : Function0<String> {
     override fun invoke() = "OK"
 }
 

compiler/testData/codegen/box/closures/closureOnTopLevel2.kt

@@ -5,7 +5,7 @@ val getter: String
 
 fun f() = { "OK" }()
 
-val obj = object : Function0<String>() {
+val obj = object : Function0<String> {
     override fun invoke() = "OK"
 }