Prioritizes coproduct remove implicits. Fixes https://github.com/milessabin/shapeless/issues/251.
Merge pull request #252 from alexarchambault/coproductremove

Prioritizes coproduct remove implicits.
Coproduct CNilOf type class
Coproduct Unite type class (reverse of Remove)
Changes in coproduct Basis
Coproduct UniteLast type class
Fixes in coproduct basis