Mixin every entity

[Quirin Schroll]

Mixins can produce types, expressions, statements, and 
declarations. That is great, but it could be better.

Why not allow mixin identifiers, storage classes, attributes, 
etc.?

Contrived example:
```d
struct mixin("C")
{
mixin("const"):
     void f(mixin("ref") int x) mixin("@safe") { }
}
```

There’s no good reason a whole entity must be written in a string 
literal with a “hole” to be filled by `format` just because its 
name is generated. Interpolated strings help, but are not 
fundamentally different.