compileToCore returned just a list of CoreBind, which isn't enough,
since to do anything with the resulting Core code, you probably also
want the type declarations. I left compileToCore as it is, but added a
function compileToCoreModule that returns a complete Core module (with
module name, type environment, and bindings). I'm not sure that
returning the type environment is the best way to represent the type
declarations for the given module, but I don't want to reinvent the
External Core wheel for this.