Allow generators to have other grammar elements as arguments

[andreas-zeller]

Allow generators to have other grammar elements as arguments. This could be used to tie abstraction layers to each other, as in

<chunk> ::= <header> <compressed_content> <trailer>;
<compressed_content> ::= <byte>* := compress(<content>);
<content> ::= <foo> <bar> <baz> := uncompress(<compressed_content>);

where <compressed_content> could thus be derived from <content>, and vice versa.