0.9.4

• Added Python 3.8 testing and support.
• Added new validate methods (moved from how-to recipes into core module):
  • Added approx() method to require for approximate numeric equality.
  • Added fuzzy() method to require strings by approximate match.
  • Added interval() method to require elements within a given interval.
  • Added set(), subset(), and superset() methods for explicit membership
    checking.
  • Added unique() method to require unique elements.
  • Added order() method to require elements by relative order.
• Changed default sequence validation to check elements by index position
  rather than checking by relative order.
• Added fuzzy-matching allowance to allow strings by approximate match.
• Added Predicate class to formalize behavior--also provides inverse-matching
  with the inversion operator (~).
• Added new methods to Query class:
  • Added unwrap() to remove single-element containers and return their
    unwrapped contents.
  • Added starmap() to unpack grouped arguments when applying a function
    to elements.
• Fixed improper use of assert statements with appropriate conditional
  checks and error behavior.
• Added requirement class hierarchy (using BaseRequirement). This gives
  users a cleaner way to implement custom validation behavior and makes
  the underlying codebase easier to maintain.
• Changed name of ProxyGroup to RepeatingContainer.
• Changed "How To" examples to use the new validation methods.