v3.3.0

Changes to ConstantTextStyle

Problem: .constant() applied the effect of .equals(()).

• .constant() now only prevents changes via .locale(_:)
• the previous behavior is now written as.constant().equals(())

Implementation details in preparation of opaque type constraints #74

• DiffableTextStyleXiOS was reduced into DiffableTextStyle
• DiffableTextViewsXiOS was split into DiffableTextViewsXiOS and DiffableTextKitXiOS