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/remove.cpp

@@ -22,10 +22,10 @@ TEST_CASE("JsonDocument::remove()") {
 
   SECTION("string literal") {
     doc["a"] = 1;
-    doc["a\0b"_s] = 2;
+    doc["x"] = 2;
     doc["b"] = 3;
 
-    doc.remove("a\0b");
+    doc.remove("x");
 
     REQUIRE(doc.as<std::string>() == "{\"a\":1,\"b\":3}");
   }