NFC: Introduce pointer domestication backend support

`capptr_domesticate<Backend>(Backend::LocalState*, CapPtr<T, B>)` is the
intended affordance for conversion to covert from a `CapPtr<T, B>` with
`B::wildness` `Wild` to a `CapPtr<>` with `B::with_wildness<Tame>` and thence
plumbed into the rest of the machinery.

David added the SFINAE wrapper so that `Backend`-s now don't need to implement a
domestication callback; instead, if the `Backend` does not provide a
`capptr_domesticate` function, a default, which just does an explicit type cast,
will be used instead.

This is not yet hooked into the rest of the tree.

Co-authored-by: David Chisnall <David.Chisnall@microsoft.com>

src/mem/localalloc.h

@@ -569,6 +569,11 @@ namespace snmalloc
 #ifdef SNMALLOC_PASS_THROUGH
       return external_alloc::malloc_usable_size(const_cast<void*>(p_raw));
 #else
+      // TODO What's the domestication policy here?  At the moment we just probe
+      // the pagemap with the raw address, without checks.  There could be
+      // implicit domestication through the `SharedStateHandle::Pagemap` or we
+      // could just leave well enough alone.
+
       // Note that this should return 0 for nullptr.
       // Other than nullptr, we know the system will be initialised as it must
       // be called with something we have already allocated.
@@ -600,6 +605,11 @@ namespace snmalloc
     void* external_pointer(void* p_raw)
     {
 #ifndef SNMALLOC_PASS_THROUGH
+      // TODO What's the domestication policy here?  At the moment we just probe
+      // the pagemap with the raw address, without checks.  There could be
+      // implicit domestication through the `SharedStateHandle::Pagemap` or we
+      // could just leave well enough alone.
+
       // TODO bring back the CHERI bits. Wes to review if required.
       MetaEntry entry =
         SharedStateHandle::Pagemap::template get_metaentry<true>(