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/add.cpp

@@ -32,6 +32,7 @@ TEST_CASE("JsonDocument::add(T)") {
     REQUIRE(doc.as<std::string>() == "[\"hello\"]");
     REQUIRE(spy.log() == AllocatorLog{
                              Allocate(sizeofPool()),
+                             Allocate(sizeofStaticStringPool()),
                          });
   }