@hackage dstring0.3.0.2

Difference strings: a data structure for O(1) append on strings. Note that a DString is just a newtype wrapper around a DList Char. The reason we need a new type instead of just a type synonym is that we can have an instance IsString DString without using language extensions (TypeSynonymInstances or FlexibleInstances) so we can write overloaded string literals of type DString.