0.3.0

• switch to associated type family for Weak inside Weaken - Strengthen
  now has Weaken as a superclass
  • I'm fairly confident that things make more sense this way - we get to
    remove an open type family, improve type inference, and prevent users from
    writing potentially dangerous instances. For that, a bit of asymmetry is
    welcome.
• better document generic derivers
• clarify instance design, provide more decomposer instances
• various refactoring