Make PrimUnlifted ShorText instance safe

[treeowl]

• Use a newtype to hide the unrestricted ByteArray
  underneath.

• Mark Data.Primitive.Unlifted.Class as Trustworthy. TODO:
  Discuss whether it's okay to do that without similarly changing
  the PrimArray instance.

Closes #22

[treeowl]

The remaining question is PrimArray. There are two approaches:

1. Consider PrimArray an inherently unsafe thing we just shouldn't worry about.
2. Imagine that someone could write a Trustworthy module exporting PrimArray, hiding all the unsafe things, and defining safe indexing functions. If we take that approach, then we need to use a similar trick to ShortText, or refrain from making the .Class module Trustworthy. I really want to make the .Class module Trustworthy, because that seems like the Right Thing.