Add 'optional' imports/exports

Author: lukewagner

design/mvp/Explainer.md

@@ -618,7 +618,9 @@ instancedecl  ::= core-prefix(<core:type>)
                 | <exportdecl>
 importdecl    ::= (import <externnamelit> <attribute>* bind-id(<externtype>))
 exportdecl    ::= (export <externnamelit> <attribute>* bind-id(<externtype>))
-externtype    ::= (<sort> (type <u32>) )
+externtype    ::= (optional <someexterntype>)
+                | <someexterntype>
+someexterntype ::= (<sort> (type <u32>) )
                 | core-prefix(<core:moduletype>)
                 | <functype>
                 | <componenttype>