pagemap: assert that entry types match

More directly ensure that a "basic" pagemap's type matches its
"concrete" pagemap parameter's entry type.  Absent this check, getting
this wrong won't be detected until even further along in template code
generation (when considering a method that sees the mismatch).

src/snmalloc/backend_helpers/pagemap.h

@@ -28,6 +28,10 @@ namespace snmalloc
      */
     using Entry = PagemapEntry;
 
+    static_assert(
+      std::is_same_v<PagemapEntry, typename ConcreteMap::EntryType>,
+      "BasicPagemap's PagemapEntry and ConcreteMap disagree!");
+
     /**
      * Instance of the concrete pagemap, accessible to the backend so that
      * it can call the init method whose type dependent on fixed_range.

src/snmalloc/ds/pagemap.h

@@ -48,6 +48,8 @@ namespace snmalloc
     size_t size{0};
 
   public:
+    using EntryType = T;
+
     /**
      * Ensure this range of pagemap is accessible
      */