Enforce that TypeAliases can only Alias a Type

[InsertCreativityHere]

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.

[InsertCreativityHere]

So, TypeAlias uses a simple TypeRef (which is really a TypeRef<dyn Type>).
Meaning this issue is bogus. TypeAlias have always only allowed types.

It just used to be that interface was a type at the Slice level, but that changed in #675.