Specify cast behavior on null descriptors (#35)

All of the new cast instructions trap if they are provided a null
descriptor value.

Author: tlively

proposals/custom-descriptors/Overview.md

@@ -371,6 +371,8 @@ return the provided reference with the type described by the descriptor.
 If the type of the descriptor is exact,
 then the type of the cast output can also be exact.
 
+If the provided descriptor is a null value, these instructions trap.
+
 ```
 ref.cast_desc reftype