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.xml

@@ -53,6 +53,7 @@
 	<entry name="first" value="0" summary="this is the first"/>
 	<entry name="second" value="1" summary="this is the second"/>
 	<entry name="third" value="2" since="2" summary="this is the third"/>
+	<entry name="negative" value="-1" since="2" summary="this is a negative value"/>
     </enum>
   </interface>
 </protocol>