Today, we use FML for core functionality such as logging and assertions in the iOS and macOS embedders. FML_LOG, FML_DLOG are implemented using C++ macros and thus currently unavailable to Swift.

Logging is platform-specific and a hook exposed via log_message_callback in the embedder API, and via Settings::log_message_callback in engine internals. We should implement a Swift-compatible logger, and use that in place of the current C++ macros.