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/backend/fixedglobalconfig.h

@@ -51,5 +51,33 @@ namespace snmalloc
     {
       Backend::init(local_state, base, length);
     }
+
+    /* Verify that a pointer points into the region managed by this config */
+    template<typename T, SNMALLOC_CONCEPT(capptr::ConceptBound) B>
+    static SNMALLOC_FAST_PATH CapPtr<
+      T,
+      typename B::template with_wildness<capptr::dimension::Wildness::Tame>>
+    capptr_domesticate(typename Backend::LocalState* ls, CapPtr<T, B> p)
+    {
+      static_assert(B::wildness == capptr::dimension::Wildness::Wild);
+
+      static const size_t sz = sizeof(
+        std::conditional<std::is_same_v<std::remove_cv<T>, void>, void*, T>);
+
+      UNUSED(ls);
+      auto address = address_cast(p);
+      auto bounds = Backend::Pagemap::get_bounds(nullptr);
+      if (
+        (address < bounds.first) || (address > bounds.second) ||
+        ((bounds.second - address) < sz))
+      {
+        return nullptr;
+      }
+
+      return CapPtr<
+        T,
+        typename B::template with_wildness<capptr::dimension::Wildness::Tame>>(
+        p.unsafe_ptr());
+    }
   };
 }