Store static strings in a dedicated pool

Because a slot id is smaller than a pointer, this change will ultimately allow reducing the slot size.

extras/tests/Misc/StringAdapters.cpp

@@ -21,7 +21,7 @@ TEST_CASE("adaptString()") {
     auto s = adaptString("bravo\0alpha");
 
     CHECK(s.isNull() == false);
-    CHECK(s.size() == 11);
+    CHECK(s.size() == 5);
     CHECK(s.isStatic() == true);
   }