add `as` support in `expose`

[usaoc]

This works like a combination of rename and expose, which is not very common, but does appear to be used in the Rhombus codebase.

Also change the grammar to disallow multiple names in a group in the multi-group form, to avoid confusion with the as form.

[mflatt]

This is ok with me.

An observation, and probably something you've already considered: If a macro implementation includes expose: $id ..., then there's a chance that the macro could be made to do something strange with three identifies for id where the middle one is as. The right implementation of the macro would be to use expose: $id; ..., I guess. This kind of subtlety shows up in a few places in Rhombus, and I don't know whether it's avoidable. At a minimum, we need to make sue that there's some way of reliably expanding, and separate groups provide that mechanisms for expose.

[usaoc]

I considered changing the grammar to expose: $id_or_rename_as; ..., but I worried about the backward compatibility. I can apply the change if that wouldn’t be a big concern.

[usaoc]

One more thing I considered and wasn’t so sure was whether the same support should be added to only. I reckoned it wouldn’t be as useful, but it would be more consistent.

[usaoc]

I applied the change and found three places where the expose: $id ... form was used (none used more than one group), among which one place was in the import-macro docs.

[mflatt]

The change to expose caused no problems with any of my other existing code, and this seems ready to merge.