Define a new function mrb_funcall_id().

`mrb_funcall_id()` takes `mrb_sym` instead of `char*` for a method name.
You can use `MRB_SYM()`/`MRB_QSYM()` to specify the method to call.

src/vm.c

@@ -401,6 +401,25 @@ mrb_funcall(mrb_state *mrb, mrb_value self, const char *name, mrb_int argc, ...)
   return mrb_funcall_argv(mrb, self, mid, argc, argv);
 }
 
+MRB_API mrb_value
+mrb_funcall_id(mrb_state *mrb, mrb_value self, mrb_sym mid, mrb_int argc, ...)
+{
+  mrb_value argv[MRB_FUNCALL_ARGC_MAX];
+  va_list ap;
+  mrb_int i;
+
+  if (argc > MRB_FUNCALL_ARGC_MAX) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "Too long arguments. (limit=" MRB_STRINGIZE(MRB_FUNCALL_ARGC_MAX) ")");
+  }
+
+  va_start(ap, argc);
+  for (i = 0; i < argc; i++) {
+    argv[i] = va_arg(ap, mrb_value);
+  }
+  va_end(ap);
+  return mrb_funcall_argv(mrb, self, mid, argc, argv);
+}
+
 static int
 ci_nregs(mrb_callinfo *ci)
 {