NFC: CapPtr in external_pointer; drop capptr_rebound

capptr_rebound was only ever going to be used for external_pointer, which now
operates entirely using pointer_offset.  So instead, just make external_pointer
use capptr::AllocWild<void>, capptr_from_client, and a new capptr_reveal_wild.

src/ds/ptrwrap.h

@@ -371,6 +371,16 @@ namespace snmalloc
     return p.unsafe_capptr;
   }
 
+  /**
+   * Like capptr_reveal, but sometimes we do mean to reveal wild pointers
+   * (specifically in external_pointer, where we're revealing something
+   * architecturally derived from a user pointer).
+   */
+  inline SNMALLOC_FAST_PATH void* capptr_reveal_wild(capptr::AllocWild<void> p)
+  {
+    return p.unsafe_capptr;
+  }
+
   /**
    * Given a void* from the client, it's fine to call it AllocWild.
    * Roughly dual to capptr_reveal().