codec.dropUnits is very slow to compile for large case classes

[mpilquist]

See http://stackoverflow.com/questions/28109303/dropunit-on-hlisted-codecs-doesnt-seem-to-work

The following case class didn't finish compiling in ~10m using 1.7.0-RC1:

import scodec._
import bits._
import codecs._
import implicits._

case class Big(
  a: Int,
  b: Int,
  c: Int,
  d: Int,
  e: Int,
  f: Int,
  g: Int,
  h: Int,
  j: Int,
  k: Int,
  l: Int,
  m: Int,
  n: Int,
  o: Int,
  p: Int
)

object Big {
  implicit val codec: Codec[Big] = (
    constant(bin"0000") :: int32 :: int32 :: int32 :: int32 :: int32 ::
    constant(bin"1111") :: int32 :: int32 :: int32 :: int32 :: int32 ::
    constant(bin"0000") :: int32 :: int32 :: int32 :: int32 :: int32
  ).dropUnits.as[Big]
}

[mpilquist]

As of 1.7.0-SNAPSHOT, above example compiles in < 2s.