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/meson.build

@@ -158,6 +158,7 @@ tests = {
 		wayland_client_protocol_h,
 		wayland_server_protocol_h,
 	],
+	'enum-validator-test': [],
 }
 
 foreach test_name, test_extra_sources: tests