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/JsonArray/add.cpp

@@ -56,6 +56,7 @@ TEST_CASE("JsonArray::add(T)") {
     REQUIRE(array[0].is<int>() == false);
     REQUIRE(spy.log() == AllocatorLog{
                              Allocate(sizeofPool()),
+                             Allocate(sizeofStaticStringPool()),
                          });
   }