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.
09bc0c6be7