Update Function Object interface

to make interface trivially copiable

There is no need to perform a copy-protected type as there is no init/clear for this type.
It is always created through an instance.

README.md

@@ -8008,7 +8008,8 @@ Example:
         {
                 sumint_t s;
                 sumint_init_with(s, 16);
-                int n = f(sumint_as_interface(s));
+                interface_t interface = sumint_as_interface(s);
+                int n = f(interface);
                 printf("sum=%d\n", n);
                 sumint_clear(s);
         }
@@ -8037,6 +8038,7 @@ It will define the following type and functions:
 
 Type representing an interface to such function object. 
 There is only one method for such type (see below).
+It is a pointer to an hidded structure and can be assigned.
 
 ##### retcode\_type name\_call(name\_t interface, type\_of\_param1, type\_of\_param 2, ...)
 

m-funcobj.h

@@ -156,7 +156,7 @@
                                                                               \
   typedef struct M_F(name, _s) {                                              \
      M_F(name, _callback_ct) callback;                                        \
-  } interface_t[1];                                                           \
+  } *interface_t;                                                             \
                                                                               \
   /* Internal type for oplist & instance */                                   \
   typedef interface_t M_F(name, _ct);                                         \
@@ -192,7 +192,7 @@
                                                                               \
   typedef struct M_F(name, _s) {                                              \
      M_F(name, _callback_ct) callback;                                        \
-  } interface_t[1];                                                           \
+  } *interface_t;                                                             \
                                                                               \
   /* Internal type for oplist & instance */                                   \
   typedef interface_t M_F(name, _ct);                                         \

tests/test-mfuncobj.c

@@ -85,7 +85,8 @@ static void test_instance11(void)
   instance11_t cmp;
   int n = 2;
   instance11_init_with(cmp, -1, STRING_CTE("INIT") );
-  int x = interface1_call(instance11_as_interface(cmp), 10, &n);
+  interface1_t interface = instance11_as_interface(cmp);
+  int x = interface1_call(interface, 10, &n);
   assert(x == -8);
   assert(string_equal_str_p(cmp->c, "CALLED"));
   instance11_clear(cmp);
@@ -111,7 +112,8 @@ static void test_instance21(void)
 {
   instance21_t cmp;
   instance21_init(cmp);
-  int x = interface2_call(instance21_as_interface(cmp));
+  interface2_t interface = instance21_as_interface(cmp);
+  int x = interface2_call(interface);
   assert(x == 1);
   instance21_clear(cmp);
 }