NFC: external_pointer address_cast earlier

Make it easier to justify our avoidance of capptr_from_client and
capptr_reveal in external_pointer by performing address_cast earlier.
In particular, with this change, we can see that the pointer (and so its
authority, in CHERI) is not passed to any called function other than
address_cast and pointer_offset, and so authority is merely propagated
and neither exercised nor amplified.

Remove the long-disused capptr_reveal_wild, which was added for earlier
versions of external_pointer.

src/test/func/memory/memory.cc

@@ -476,7 +476,7 @@ void test_remaining_bytes()
     char* p = (char*)alloc.alloc(size);
     for (size_t offset = 0; offset < size; offset++)
     {
-      auto rem = alloc.remaining_bytes(p + offset);
+      auto rem = alloc.remaining_bytes(address_cast(pointer_offset(p, offset)));
       if (rem != (size - offset))
       {
         printf(