alias and mixin

[Philippe Sigaud via Digitalmars-d-learn]

> It is basically just an annoying grammar limitation that does not allow to
> use mixin / __traits as an identifier.

The usual helper template:

```
    alias helper(alias a) = a;
```

helps for aliasing __traits and anonymous function templates (x =>
x+1), but I don't see an easy solution for mixin inside the current
language, apart from pushing the mixin outside.