PagemapRegisterRange: don't presume Pagemap entry type

To date, we've had exactly one kind of Pagemap and it held exactly one
type of thing, a descendant of class MetaEntryBase.

PagemapRegisterRange tacitly assumed that the Pagemap (adapter) it
interacted would therefore store entries that could have .set_boundary()
called on them.  But in general there's no requirement that this be
true; Pagemaps are generic data structures.

To enable reuse of the PagemapRegisterRange machinery more generally,
change the type of Pagemap::register_range() to take a pointer (rather
than an address) and move the MetaEntryBase-specific functionality to
the backend_helpers/pagemap adapter.

src/test/func/domestication/domestication.cc

@@ -36,7 +36,7 @@ namespace snmalloc
     using LocalState = StandardLocalState<
       Pal,
       Pagemap,
-      Pipe<PalRange<Pal>, PagemapRegisterRange<Pagemap, false>>>;
+      Pipe<PalRange<Pal>, PagemapRegisterRange<Pagemap>>>;
 
     using GlobalPoolState = PoolState<CoreAllocator<CustomConfig>>;