Drop KFunctionFromReferenceImpl, make FunctionImpl an interface

23 invokes in KFunctionFromReferenceImpl (and consequently, in FunctionImpl)
were needed before 1576160390: a wrapped function
reference must have had the necessary invoke to be called as an instance of a
specific function type. After 1576160390, this is
not needed anymore because KFunctionFromReferenceImpl is now an internal
implementation detail of reflection, and no invoke is ever called on it.

compiler/testData/codegen/box/javaInterop/lambdaInstanceOf.kt

@@ -0,0 +1,23 @@
+// WITH_RUNTIME
+// FILE: J.java
+
+import kotlin.Function;
+import kotlin.jvm.functions.Function0;
+import kotlin.jvm.functions.Function1;
+import kotlin.jvm.functions.Function2;
+
+public class J {
+    public static String test(Function<String> x) {
+        if (x instanceof Function1) return "Fail 1";
+        if (x instanceof Function2) return "Fail 2";
+        if (!(x instanceof Function0)) return "Fail 3";
+
+        return ((Function0<String>) x).invoke();
+    }
+}
+
+// FILE: K.kt
+
+fun box(): String {
+    return J.test({ "OK" })
+}