Skip a redundant check for the rb_obj_hash case

Also, refactor the cfunc struct to use a new rb_cfunc_t.

hash.c

@@ -220,12 +220,12 @@ obj_any_hash(VALUE obj)
     VALUE klass = CLASS_OF(obj);
     if (klass) {
         const rb_callable_method_entry_t *cme = rb_callable_method_entry(klass, id_hash);
-        if (cme && METHOD_ENTRY_BASIC(cme) && RBASIC_CLASS(cme->defined_class) == rb_mKernel) {
+        if (cme && METHOD_ENTRY_BASIC(cme)) {
             // Optimize away the frame push overhead if it's the default Kernel#hash
-            if (cme->def->type == VM_METHOD_TYPE_CFUNC && cme->def->body.cfunc.func == (VALUE (*)(ANYARGS))rb_obj_hash) {
+            if (cme->def->type == VM_METHOD_TYPE_CFUNC && cme->def->body.cfunc.func == (rb_cfunc_t)rb_obj_hash) {
                 hval = rb_obj_hash(obj);
             }
-            else {
+            else if (RBASIC_CLASS(cme->defined_class) == rb_mKernel) {
                 hval = rb_vm_call0(GET_EC(), obj, id_hash, 0, 0, cme, 0);
             }
         }

method.h

@@ -136,8 +136,9 @@ typedef struct rb_method_iseq_struct {
     rb_cref_t * cref;          /*!< class reference, should be marked */
 } rb_method_iseq_t; /* check rb_add_method_iseq() when modify the fields */
 
+typedef VALUE (*rb_cfunc_t)(ANYARGS);
 typedef struct rb_method_cfunc_struct {
-    VALUE (*func)(ANYARGS);
+    rb_cfunc_t func;
     VALUE (*invoker)(VALUE recv, int argc, const VALUE *argv, VALUE (*func)(ANYARGS));
     int argc;
 } rb_method_cfunc_t;