Allow hosts to call into scripts

[velipso]

I've been stuck thinking about closures (see #51), and eventually rejected them.

It dawned on me that I could support exporting functions to the host environment without using closures, which would remove the need of an event loop.

Perhaps something like this:

def onMouseMove x, y : "company.product.onMouseMove"
  # ...
end

The : "company.product.onMouseMove" means that the command will be exported to the host using that name. The name will be hashed into a 64-bit number, just like native commands.

This means inside the host, you could do something like:

sink_val args = sink_list_newempty(ctx);
sink_list_push(ctx, args, sink_num(123));
sink_list_push(ctx, args, sink_num(456));
sink_val result = sink_ctx_call(ctx, "company.product.onMouseMove", args);

The sink_ctx_call function would hash the string (alternatively could use sink_ctx_callhash so symbols could be stripped), then execute the function, and get the result.

This is effectively the reverse of the native function API, which seems very logical and useful.