Struct member attributes

[bvdberg]

Sometimes it's useful to have explicit padding members in a struct. These are usually unused, so they might generate a warning. Currently we dont allow having Attributes on struct members. When implementing attributes (unused attr for now), I ran into the issue of comma separated members, like:

   i32 a, b, c;

When adding the @(..) at the and, it should apply to all, but implementing this is nasty, since we would then need to remember all decls so far. Another option is to only allow attributes on single-decl members (ie not using the comma)..

[chqrlie]

I think attributes should only apply to the member defined just before the @, as is the case for function arguments, the implementation should be simple.

It might be considered bad style to use attributes in definitions with multiple items, I am OK with rejecting this.