Better handling of higher-order functions

Function literals may confuse the plugin. For instance,

```
  val xs = List(1, 2, 3)
  xs map (x => x)
```

is translated to 

```
  val xs = List(1, 2, 3)
  xs map (new anonfun extends AbstractFunction1$II {
    def apply(x: Object) = apply(x.toInt) // boxing
    def apply(x: Int) = x
  })
```

but, if `map` is specialized, no boxing will occur (the `apply(Object)` overload is never called).
