Enforce that TypeAliases can only Alias a Type #730
Description
As pointed out by #452 (comment), we currently allow you to typealias an interface. In the past, we were pretty loosey-goosey with what the 'type' in 'typealias' meant, but now that we're solidified it (and made interface no longer a type), we should just disallow aliasing an interface.
We should also check if there are any other Slice elements which are valid in a type-alias, but are not types.
These should also be rejected.