bettersum (v0.7.0): mixin-based sumtypes

[DanielG]

Oh, and if you're concerned about compilation speed: quote your D 
types in the sumtype definition with backticks - it will 
short-circuit most of the parsing:

```d
mixin(sumtype(q{
Thing {
   // backticks for speed:
   Value1(`int function(const(int*)[], immutable(int[int])*)[]` 
whatever)
}
}));
```

The reason I made it parse D types at all, is so they could 
appear normally in `q{}` strings, and benefit from syntax 
highlighting. But if you don't care about that - backtick-quote 
away!