CDDL Module Structure

At the time of writing, the Concise Data Definition Language (CDDL) is defined by RFC 8610 and RFC 9682 as well as RFC 9165 and RFC 9741. The latter two have used the extension point provided in RFC 8610, the _control operator_. As CDDL is being used in larger projects, the need for features has become known that cannot be easily mapped into this single extension point. The present document defines a backward- and forward-compatible way to add a module structure to CDDL.