Add 'optional' imports/exports

Author: lukewagner

design/mvp/Binary.md

@@ -232,7 +232,9 @@ instancedecl  ::= 0x00 t:<core:type>                      => t
                 | 0x04 ed:<exportdecl>                    => ed
 importdecl    ::= na:<nameattributes> et:<externtype>     => (import na et)
 exportdecl    ::= na:<nameattributes> et:<externtype>     => (export na et)
-externtype    ::= 0x00 0x11 i:<core:typeidx>              => (core module (type i))
+externtype    ::= 0x10 t:<someexterntype>                 => (optional t)
+                | t:<someexterntype>                      => t
+someexterntype ::= 0x00 0x11 i:<core:typeidx>             => (core module (type i))
                 | 0x01 i:<typeidx>                        => (func (type i))
                 | 0x02 b:<valuebound>                     => (value b) 🪙
                 | 0x03 b:<typebound>                      => (type b)
@@ -541,6 +543,7 @@ named once.
   subset of `canonopt`s.
 * Add optional `shared` immediate to all canonical definitions (explicitly or
   via `<canonopt>`) when shared-everything-threads (🧵②) is added.
+* Improve `externtype` opcodes for `optional`
 
 
 [`core:byte`]: https://webassembly.github.io/spec/core/binary/values.html#binary-byte

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>