Reinstated some Unifier tests; added (failing) test for unifying invariant type constructors.
Reinstated some Unifier tests; added (failing) test for unifying invariant type constructors.

notxcain/shapeless,alexarchambault/shapeless,khronnuz/shapeless,joescii/shapeless,TomasMikula/shapeless,roberth/shapeless,tek/shapeless,wheaties/shapeless,Atry/shapeless,lambdista/shapeless,triggerNZ/shapeless,fthomas/shapeless,malcolmgreaves/shapeless,liff/shapeless,milessabin/shapeless,clhodapp/shapeless,isaka/shapeless,lukasz-golebiewski/shapeless