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/pagemap.h

@@ -205,6 +205,15 @@ namespace snmalloc
       body_opt = new_body;
     }
 
+    template<bool has_bounds_ = has_bounds>
+    std::enable_if_t<has_bounds_, std::pair<address_t, size_t>> get_bounds()
+    {
+      static_assert(
+        has_bounds_ == has_bounds, "Don't set SFINAE template parameter!");
+
+      return {base, size};
+    }
+
     /**
      * Get the number of entries.
      */