scanner: add validators for enums

Right now compositors need to manually check that enum values sent
by the client are valid. In particular:

- Check that the value sent by the client is not outside of the enum.
- Check that the version of the enum entry is consistent with the
  object version.

Automatically generate validator functions to perform these tasks.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: https://gitlab.freedesktop.org/wayland/wayland/-/issues/104

tests/data/small-private-code.c

@@ -25,6 +25,7 @@
  * SOFTWARE.
  */
 
+#include <stdbool.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include "wayland-util.h"