• Added: Finally implement Type::Coercion's has_coercion_for_type method.
  
• Added: Type::Tiny equals/is_subtype_of/is_supertype_of/is_a_type_of
  methods for type constraint comparisons.
  
• Added: Type::Tiny plus_coercions/minus_coercions/no_coercions methods
  for creating subtypes with different sets of coercions.
  
• Allow coercion code to be expressed as a string; quite a bit faster.
  
• Create and use compiled coercions; somewhat faster.