Declaring sigs and structs:

<decls> ::= ...
<sigbody> ::= <decls> | %include <signame>. <sigbody>
<sig> ::= { <sigbody> }
<sigdecl> ::= %sig <signame> = <sig>. | %sig <signame> = <signame>.
<structdecl> ::=   %struct <structname> : <signame>.
                 | %struct <structname> = <structname>.
                 | %struct <structname> : <sig>.