Nitpick: if we want to stay with our current `std.traits`'s scheme, this should be called `isSwitchableEnum`.

Or just `isSwitchable` actually, the implementation can work for anything...
```D
static assert( isSwitchable!int);
static assert(!isSwitchable!double);
```

I know it's just a private helper, but we might as well write it as if it was destined for public use.