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/JsonDocument/ElementProxy.cpp

@@ -31,6 +31,7 @@ TEST_CASE("ElementProxy::add()") {
     REQUIRE(doc.as<std::string>() == "[[\"world\"]]");
     REQUIRE(spy.log() == AllocatorLog{
                              Allocate(sizeofPool()),
+                             Allocate(sizeofStaticStringPool()),
                          });
   }