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/constructor.cpp

@@ -64,6 +64,7 @@ TEST_CASE("JsonDocument constructor") {
     REQUIRE(doc2.as<std::string>() == "{\"hello\":\"world\"}");
     REQUIRE(spyingAllocator.log() == AllocatorLog{
                                          Allocate(sizeofPool()),
+                                         Allocate(sizeofStaticStringPool()),
                                      });
   }
 
@@ -87,6 +88,7 @@ TEST_CASE("JsonDocument constructor") {
     REQUIRE(doc2.as<std::string>() == "[\"hello\"]");
     REQUIRE(spyingAllocator.log() == AllocatorLog{
                                          Allocate(sizeofPool()),
+                                         Allocate(sizeofStaticStringPool()),
                                      });
   }