add a note about 0_usize and null

Author: nikomatsakis

active_discussion/representation.md

@@ -149,6 +149,8 @@ To start, we will create threads for each major categories of types
     - When using the C ABI, these map to the C pointer types, presumably
 - Raw pointers
     - Effectively same as integers?
+    - Is `ptr::null` etc guaranteed to be equal in representation to `0_usize`?
+      - C does guarantee that `0` when cast to a pointer is NULL
 - Representation knobs:
     - Custom alignment ([RFC 1358])
     - Packed ([RFC 1240] talks about some safety issues)