fix_polymorhic_constructors.md

• Defer monomorphization for data constructors

  In 0.15.4 and earlier, the compiler monomorphizes type constructors early, yielding the following type:

  > :t Nothing
  forall (a1 :: Type). Maybe a1
  
  > :t { a : Nothing }
  forall (a1 :: Type).
    { a :: Maybe a1
    }

  With this change, the monomorphization introduced in #835 is deferred to only when it's needed, such as when constructors are used as values inside of records.

  > :t Nothing
  forall a. Maybe a
  
  > :t { a : Nothing }
  forall (a1 :: Type).
    { a :: Maybe a1
    }