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/enum-validator-test.c

@@ -0,0 +1,13 @@
+#include <assert.h>
+#include "data/small-server-core.h"
+
+int
+main(int argc, char *argv[]) {
+	assert(intf_A_foo_is_valid(INTF_A_FOO_FIRST, 1));
+	assert(intf_A_foo_is_valid(INTF_A_FOO_FIRST, 2));
+
+	assert(!intf_A_foo_is_valid(INTF_A_FOO_THIRD, 1));
+	assert(intf_A_foo_is_valid(INTF_A_FOO_THIRD, 2));
+
+	assert(intf_A_foo_is_valid(INTF_A_FOO_NEGATIVE, 2));
+}