Pattern match optimization.

[sir-wabbit]

In scalaz/scalaz#1958 @fommil uses this optimization:

fa match {
  case Right(b) => f(b)
  case Left(a) => Left(a)
}

fa match {
  case Right(b) => f(b)
  case a => a.asInstanceOf[Either[L, B]]
}

perhaps we could detect this particular case and optimize it automatically.