First runtime performance benchmarks.

Review by @gzm0
Ah, because the ScalaMeter framework provides us with field positions at runtime. 
The created body is:
```scala
in { pos: Int => pos match {
 case 1 => iterations foreach { _ => rec.f1 }
 case 2 => iterations foreach { _ => rec.f2 }
}
```
The switch is inserted to jump to the right implementation.