Fix some comments

Fixing some comments. Especially those that contain ' ' or " ", which
are special markup in Haddock indicating a name or a module. Replacing
these with /…/ for italic text and @…@ for typewriter text.
Add Unpacker to exports (without constructor)

'unpackEquals' is exported from "Language.Scheme.Primitives" and it
needs 'Unpacker' for its type. However, 'Unpacker' has not been
exported.