Macro zaplib::register_call_rust

macro_rules! register_call_rust {
    ( $ call_rust: ident) => { ... };
}

Define state-less entry point

This can be used when application state and canvas drawing are not needed, but instead just JavaScript to Rust communications. TODO(Paras): This is a bit of hack right now, since we end up instantiating a lot of Zaplib features that end up unused. We should split out the framework in the future so that thisuse case does not initialize unused parts of Zaplib as well as JavaScript mouse and keyboard event handlers.