Make FunctionN interfaces

FunctionN and ExtensionFunctionN are now interfaces, with the corresponding
implementations in FunctionImplN and ExtensionFunctionImplN

Update test data

compiler/testData/diagnostics/tests/override/ExtendFunctionClass.kt

@@ -1,9 +1,9 @@
 package extendFunctionClass
 
-class <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>A<!> : Function1<Int, Int>() {
+class <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>A<!> : Function1<Int, Int> {
 
 }
 
-class B : Function1<Int, Int>() {
+class B : Function1<Int, Int> {
     override fun invoke(p1 : Int) = p1
 }